Reducing scintillation noise in free space optical communications

ABSTRACT

In some embodiments, an optical communication system may include an optical source, a modulator, and a photoreceiver. The optical source may be configured to generate a beam comprising a series of light pulses each having a duration of less than 100 picoseconds. The photoreceiver may have a detection window duration of less than 1 nanosecond. When a first pulse travels through a variably refractive medium, photons in the first pulse may be refracted to travel along different ray paths to arrive at the photoreceiver according to a temporal distribution curve. A full width at half maximum (FWHM) value of the temporal distribution curve may be at least three times as large as a coherence time value of the first pulse, and the detection window of the photoreceiver may be at least six times as large as the FWHM value of the temporal distribution curve.

FIELD OF THE DISCLOSURE

The subject matter described herein relates to free-space optical (FSO)wireless transmission including optical communications, remote-sensing,laser ranging, power beaming, etc., and more particularly, to enhancedoptical transport efficiencies that can be realized for wavelengthpropagation using ultra-short-pulse-laser (USPL) sources for beampropagation through a variably refractive medium such as the Earth'satmosphere.

BACKGROUND

FSO communications have potential to greatly increase data throughput,decrease cost, and increase access for high-speed internet and othercommunications technologies. To date, however, FSO communication systemshave had limited operational success due to atmospheric interference,which reduces the distance over which data can be optically transmittedand introduces bit errors. Meanwhile, alternative communicationstechnologies, such as radiofrequency and microwave communications, facesignificant spectrum limitations and cannot be used to deliversufficient data to meet demand. Currently available optical systems arenot able to produce sufficiently accurate, reliable, and available datatransmission results that can reliably offload communications demandfrom these radiofrequency and microwave systems and improve datatransmission and access, nor can currently available optical systemstransmit data over long distances.

Accordingly, there is a need for optical communication systems that canprovide highly reliable, highly available data transmission over longdistances. Further, there is a need for optical communication that canreliably transmit data over long distances, such as half a mile or more.

SUMMARY

The following description presents a simplified summary in order toprovide a basic understanding of some aspects described herein. Thissummary is not an extensive overview of the claimed subject matter. Itis intended to neither identify key or critical elements of the claimedsubject matter nor delineate the scope thereof.

In some embodiments, an optical communication system for opticallytransmitting data through a variably refractive medium may include anoptical source, a modulator, and a photoreceiver. The optical source maybe configured to generate a beam comprising a series of light pulseseach having a duration of less than 100 picoseconds. The modulator maybe configured to modulate the series of light pulses in response to adata transmission signal, thereby encoding transmission data into theseries of light pulses. The photoreceiver may have a detection windowduration of less than 1 nanosecond and a detection threshold. Thephotoreceiver may be configured to indicate whether a received opticalenergy during a given detection window is greater than the detectionthreshold. The series of light pulses may include a first light pulsehaving a coherence length of less than 400 microns. When the first pulsetravels through the variably refractive medium, photons in the firstpulse may be refracted to travel along different ray paths havingdifferent lengths to the photoreceiver, and the photons of the firstpulse may arrive at the photoreceiver according to a temporaldistribution curve that depends, at least in part, on the duration ofthe first pulse and the lengths of the different ray paths taken by thephotons in the first pulse to the photoreceiver. A full width at halfmaximum (FWHM) value of the temporal distribution curve may be at leastthree times as large as a coherence time value equal to the coherencelength of the first pulse divided by the speed of light through thevariably refractive medium, and the detection window of thephotoreceiver may be at least six times as large as the FWHM value ofthe temporal distribution curve.

In some embodiments, a laser ranging system may include an opticalsource and a photoreceiver. The optical source may be configured togenerate a beam comprising a series of light pulses each having aduration of less than 100 picoseconds. The photoreceiver may have adetection window duration of less than 1 nanosecond and a detectionthreshold. The photoreceiver may be configured to indicate whether areceived optical energy during a given detection window is greater thanthe detection threshold. The series of light pulses may include a firstlight pulse having a coherence length of less than 400 microns. When thefirst pulse travels through the variably refractive medium, photons inthe first pulse may be refracted to travel along different ray pathshaving different lengths to the photoreceiver. The photons of the firstpulse may arrive at the photoreceiver according to a temporaldistribution curve that depends, at least in part, on the duration ofthe first pulse and the lengths of the different ray paths taken by thephotons in the first pulse to the photoreceiver. A full width at halfmaximum (FWHM) value of the temporal distribution curve may at leastthree times as large as a coherence time value equal to the coherencelength of the first pulse divided by the speed of light through thevariably refractive medium, and the detection window of thephotoreceiver may be at least six times as large as the FWHM value ofthe temporal distribution curve. The laser ranging system may beconfigured to transmit the series of light pulses toward a surface,receive at least a portion of the series of light pulses that have beenreflected by the surface, and, based on a time of flight of the receivedportion of the series of light pulses, determine a distance of at leasta portion of the surface from the laser ranging system.

Further variations encompassed within the systems and methods aredescribed in the detailed description of the invention below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, show certain aspects of the subject matterdisclosed herein and, together with the descriptions, help explain someof the principles associated with the disclosed implementations.

FIG. 1 depicts an example of an optical communications platformincluding free-space coupling of a USPL source as an optical source fortransport to a remote optical receive terminal.

FIG. 2 depicts an example of an optical communications platformincluding fiber coupling of a USPL source as an optical source fortransport to a remote optical receive terminal.

FIG. 3 depicts an example of an optical communications platformincluding fiber coupling of a USPL source to an external modulator fortransport to a remote optical receive terminal.

FIG. 4 depicts an example of an optical communications platformincluding fiber coupling of a USPL source to an external modulatorthrough a fiber medium for transport to a remote optical receiveterminal.

FIG. 5 depicts an example of a transmitting and or receiving elements,which can be of a type from either the Hyperbolic Mirror FabricationTechniques or conventional Newtonian designs.

FIG. 6 depicts an example of an optical fiber amplifier elementidentified and used to increase enhancing optical transmit launch powerfor transport to a remote optical receive terminal.

FIG. 7 depicts an example of a USPL laser device that is fiber coupledto an external modulator for transport, in a point-to-pointconfiguration for transport to a remote optical receive terminal.

FIG. 8 depicts an example of a USPL laser device that is fiber coupledto an external modulator for transport, in a point-to-multi-pointconfiguration.

FIG. 9 depicts an example of use of USPL sources acting as tracking andalignment (pointing) beacon sources.

FIG. 10 depicts an example of a USPL laser sources polarizationmultiplexed onto a transmitted optical signal, to provide PolarizationMultiplex USP-FSO (PM-USP-FSO) functionality.

FIG. 11A and FIG. 11B respectively depict examples of USPL-FSOtransceivers utilized for use in line-of-sight and non-line-of-sightlasercom applications.

FIG. 12 depicts an example of light including light from the data signalpropagated forward being backscattered by interaction with air-borneparticulates that are the subject of investigation.

FIG. 13 depicts an example of USPL laser sources as optics receptiontechniques to improve detection sensitivity; USPL laser sources as wellas optical reception techniques to improve detection sensitivity.

FIG. 14 depicts an example of a USPL-FSO transceiver utilized andoperated across the infrared wavelength range optionally including lightfrom the data signal as a range-finder and spotting apparatus for thepurposes of target identification.

FIG. 15 depicts an example of a USPL pulse multiplier device consistentwith implementations of the current subject matter.

FIG. 16 depicts another example of a device for generation of high pulserate USPL optical streams consistent with implementations of the currentsubject matter.

FIG. 17 depicts another example of an optical device to a generate aUSPL RZ data stream from a conventional transmission networking element.

FIG. 18 depicts an example of a implementing a USPL pulse multiplierdevice for generation of 10×TDM type signals system to give a 100 Gbpsoutput.

FIG. 19 depicts an example of a implementing another type of USPL pulsemultiplier device for extending the pulse repetition rate for use inhigh capacity networks.

FIG. 20 depicts an example of a implementing another type of USPL pulsemultiplier device for extending the pulse repetition rate for use inhigh capacity networks.

FIG. 21 depicts examples of active mode-locked linear fiber lasers withfeedback regenerative systems: fiber reflector (FR), wavelength-divisionmultiplexer (WDM), Erbium-doped fiber (EDF), optical coupler (OC),photo-detector (PD), phase-locked loop (PLL), and Mach-Zehnder Modulator(MZM).

FIG. 22 and FIG. 23 depict examples of passive mode-locked linear fiberlasers using a carbon nano-tubes saturable absorber: fiber reflector(FR), wavelength-division multiplexer (WDM), Erbium-doped fiber (EDF),optical coupler (OC), and saturable absorber (SA).

FIG. 24 depicts an example of a time-delay stabilization mechanism:optical coupler (OCin, OCout), photo-detector (PDin, PDout), high-passfilter (HPF), low-pass filter (LPF), phase-locked loop (PLL),phase-comparator (PC), frequency-divider (1/N), clock-data recoverysystem (CDR), piezoelectric actuator (PZ1 . . . PZN), summing op amp,for use in stabilizing the optical pulse to pulse relationship producedfrom the USPL source.

FIG. 25A and FIG. 25B respectively include a schematic diagram and agraph relating to an example of a controlling mechanism to stabilize theoutput frequency of TDM sources utilizing an idealized PZ actuator.

FIG. 26 depicts an example of a Time-Domain Multiplexing (TDM) where theTDM multiplexes a pulse train using parallel time delay channels, havingthe delay channels to be “consistent” relative to each another (Becausethe frequency of an output multiplexed pulse train is ideally asinsensitive as possible to environmental changes, a feedback loopcontrol system can correct the delay units for any fluctuations whichcompromise the stability of the output rep rate, and feedback can beprovides through interconnection to a Neural Network).

FIG. 27 depicts an example of use of fiber based collimators along withPiezoelectric transducers for controlling individual MFC circuits.

FIG. 28 depicts an example of timing of the TDM chip from the USPLmodulation source to provide a Terabit/second (or faster) with aMultiplier Photonic chip.

FIG. 29 depicts an example of timing of the TDM chip from the USPLmodulation source to provide a Terabit/second (or faster) with aMultiplier Photonic chip operating in a WDM configuration.

FIG. 30 depicts an example of construction of a computer assistedsystem, which can control the pulse width of an all-fiber mode-lockedlaser using recursive linear polarization adjustments with simultaneousstabilization of the cavity's repetition rate using a synchronousself-regenerative mechanism and can also offer tunability of therepetition rate, and pulse width.

FIG. 31 depicts an example of a modified pulse interleaving scheme, by apulse multiplication technique, in which the lower repetition rate pulsetrain of a well-characterized, well-mode locked laser can be coupledinto an integrated-optical directional coupler, where a well-determinedfraction of the pulse is tapped off and “re-circulated” in an opticalloop with an optical delay equal to the desired inter-pulse spacing inthe output pulse train, and re-coupled to the output of the directionalcoupler.

FIG. 32 is a process flow chart illustrating features of a methodconsistent with implementations of the current subject matter.

FIG. 33 is another process flow chart illustrating features of a methodconsistent with implementations of the current subject matter.

FIG. 34 is another process flow chart illustrating features of a methodconsistent with implementations of the current subject matter.

FIGS. 35A and 35B show exemplary nodes that can be used for transmittingand/or receiving information.

FIG. 36 shows an exemplary arrangement in which data is transmitted froma first communications network 3542 to a second communications network3544 over an optical communication distance D using a transmit node 3510and a receiving node 3530.

FIG. 37 shows an exemplary beam traveling over an optical communicationdistance D, such as 1 mile, through a constant refractive medium.

FIG. 38 provides a diagrammatic representation of photons in a beamtraveling through a variably refractive medium.

FIG. 39 shows a diagrammatic representation of a pulse broadening as ittravels over an optical communication distance.

FIG. 40 shows an exemplary temporal distribution curve of ashort-duration pulse 4010 that traveled a substantial distance through avariably refractive medium and has been temporally broadened.

FIG. 41 shows a diagrammatic representation of light pulses arriving indetection windows of a photoreceiver.

FIG. 42 shows an example of test data received over a one-mile opticalcommunication distance.

FIG. 43 shows an exemplary ranging node that can be used to detectobjects or surfaces and determine positions of those objects relative tothe node.

DETAILED DESCRIPTION

While aspects of the subject matter of the present disclosure may beembodied in a variety of forms, the following description andaccompanying drawings are merely intended to disclose some of theseforms as specific examples of the subject matter. Accordingly, thesubject matter of this disclosure is not intended to be limited to theforms or embodiments so described and illustrated.

FIG. 1 illustrates an example of an optical communications platform 100configured to use an USPL source as an optical source for transport. Asshown in FIG. 1 , a USPL source 102 may be directly modulated by anexternal source element 104. Optical power from the USPL source 102 canbe coupled across free space 110 to a transmitting element 106,optionally by an optical telescope. The transmitting element 106 canoptionally include optical components formed by hyperbolic mirrorfabrication techniques, conventional Newtonian designs, or the like. Areciprocal receiving telescope at a receiver system can provide foroptical reception. Consistent with implementations of the currentsubject matter, each optical transport platform can be designed tooperate as a bi-directional unit. In other words, the transmittingelement 106 of the optical communications platform 100 can also functionas a receiving element. In general, unless otherwise explicitly stated,a transmitting element 106 as described can be considered to also befunctional as a receiving element and vice versa. An optical elementthat performs both transmission and receiving functions can be referredto herein as an optical transceiver.

FIG. 2 illustrates an example of an optical communications system 200that includes the optical communications platform 100 of FIG. 1 . Alsoshown in FIG. 2 is a second complementary receiving element 204, whichcan be a receiving telescope located at a remote distance from thetransmitting element 106. As noted above, both the transmitting element106 and the receiving element 204 can be bi-directional, and each canfunction as both a transmitting element 106 and a receiving element 204depending on the instantaneous direction of data transmission in theoptical communications system 200. This feature applies throughout thisdisclosure for transmitting and receiving elements unless otherwiseexplicitly stated. Either or both of the transmitting element 106 andthe receiving element 204 can be optical telescopes or other devices fortransmitting and receiving optical information.

FIG. 3 illustrates an example of an optical communications platform 300for using an USPL source 102 fiber coupled to an external modulator 302through a fiber medium 304 and connected to a transmitting element 106through an additional transmission medium 306, which can optionally be afiber medium, a free space connection, etc. The USPL source 102 can beexternally modulated by the external modulator 302 such that opticalpower from the USPL source 102 is fiber coupled to the transmittingelement 106 or handled via an equivalent optical telescope.

FIG. 4 illustrates an example of an optical communications system 400that includes the optical communications platform 300 of FIG. 3 . Alsoshown in FIG. 4 is a second complementary receiving telescope 204,which, as noted above in relation to FIG. 2, can be a receivingtelescope located at a remote distance from the transmitting element106.

FIG. 5 illustrates an example of an optical communications architecture500. The architecture 500 of FIG. 5 may include the elements of FIG. 4and may further include a first communication network 502 connected to afirst optical communications platform 300. The receiving element 204 ispart of a second optical communications platform 504, which canoptionally include components analogous to those of the first opticalcommunications platform 300. A second communications network 506 can beconnected to the second optical communications platform 504 such thatthe data transmitted optically between the transmitting element 106 andthe receiving element 204 or are passed between the first and secondcommunications networks 502, 506, which can each include one or more ofoptical and electrical networking features.

FIG. 6 illustrates an example of an optical communications system 600.As part of an optical communications platform 602, an USPL source 102 isfiber coupled to an external modulator 302, for example through anoptical fiber 202 or other transmission medium. The light from the USPLsource 102 is propagated via a transmitting element 106 in a similarmanner as discussed above. An optical amplifier element 604, which canoptionally be an optical fiber amplifier element, can be used toincrease optical transmit launch power, and can optionally be disposedbetween the external modulator 302 and the transmitting element 106 andconnected to one or both via an additional transmission medium 306,which can optionally be a fiber medium, a free space connection, etc.Also shown in FIG. 6 is a second complementary receiving element 204located at a remote distance from the optical communications platform602. It will be readily understood that a second optical communicationsplatform 504 that includes the receiving element 204 can also include anoptical amplifier element 604. First and second communications networks502, 506 can be connected respectively to the two optical communicationsplatforms 602, 504.

FIG. 7 illustrates an example of an optical communications system 700.The optical communications platform 602 shown in FIG. 6 can be incommunication with a second optical communications platform 702, whichcan in this implementation include a receiving element 204 and anoptical preamplifier 704. Other components similar to those shown in theoptical communications platform 602 can also be included in the secondoptical communications platform 702, although they are not shown in FIG.7 . It will be understood that a bi-directional optical communicationsplatform can include both of an optical preamplifier 704 for amplifyinga received optical signal and an optical amplifier element 604 forboosting a transmitted optical signal.

Consistent with the implementation depicted in FIG. 7 and otherimplementations of the current subject matter, optical amplification(e.g. for either or both of an optical amplifier element 604 or anoptical preamplifier 704) be included for enhancing the optical budgetfor the data-link between the transmitting element 106 and the receivingelement 204 (and vice versa), for example using one or more of anerbium-doped fiber amplifier (EDFA), a high power erbium-ytterbium dopedfiber amplifier (Er/Yb-DFA), or equivalents, which can include but arenot limited to semiconductor-optical-amplifiers (SOA).

FIG. 8 illustrates an example of an optical communications system 800.The optical communications platform 602 shown in FIG. 6 can be incommunication with a second optical communications platform 802, whichcan in this implementation include a receiving element 204 and anoptical preamplifier 704 similar to those shown in FIG. 7 . As shown inFIG. 8 , the second optical communications platform 802 can furtherinclude optical receiver circuitry 804, which can receive amplified andelectrically recovered data received at the receiving element 204 andamplified by the optical preamplifier. A plurality of clock sources 806can interface to multiple remote multi-point network connections with aplurality of communications networks 810 as required. In a similarmanner, a complementary set of clock sources and multiple communicationnetworks can be operated in conjunction with the optical communicationsplatform 602 (e.g. in place of the single depicted communication network502 in FIG. 8 ).

FIG. 9 illustrates an example of an optical communications system 900.An optical communications platform 902, which can feature similarelements to those in the optical communications platform 602 firstdiscussed herein in reference to FIG. 6 , can also include an additionalUSPL source 904 acting as a tracking and alignment (pointing) beaconsource. A second optical communications platform 906 can also include anadditional USPL source 910 acting as a tracking and alignment (pointing)beacon source. The tracking and alignment (pointing) beacon sources 904,910 can optionally originate from available communications sources usedin data transport transmission, or can be provided by separate,dedicated USPL sources. In addition, each USPL beacon source 904, 910can include an in-band or out-of-band source, thereby allowing theadvantage of available optical amplification sources, or from dedicatedoptical amplification resources.

FIG. 10 illustrates an example of a FSO communication system 1000 thatincludes a dual polarization USPL-FSO optical data-link platform 1001 inwhich USPL sources are polarization multiplexed onto a transmittedoptical signal to thereby provide polarization multiplexed USP-FSO(PM-USP-FSO) functionality. Two USPL sources 102 and 1002 are fibercoupled to either directly modulated or externally modulated modulationcomponents 1004, 1006 respectively. Each respective modulated signal isoptically amplified by an optical amplifier component 1010, 1012followed by adjustment of optical polarization states using polarizationcomponents 1014, 1016. The polarization state signals are fiber coupledto a polarization dependent multiplexer (PDM) component 1020 forinterfacing to an optical launch platform component 1022, which can besimilar to the transmit element 106 discussed above. The PDM 1020multiplexes the light of differing polarization states into a singlepulse train for transmission via the optical launch platform component1022. An USPL optical beacon 904 can be included to provide capabilitiessimilar to those discussed above in reference to FIG. 9 , for example tooperate along or in conjunction with a second USPL optical beacon 906 ata receiving platform 1024, which can include a receiving element 204similar to those described above. As previously noted, the receivingelement 204 as well as other features and components of the receivingplatform 1024 can generally be capable of supporting transmissionfunctions such that a bi-directional link is established. A receivedsignal recovered by the receiving element 204 can provide an opticalsignal that is interfaced to an appropriate polarization dependentde-multiplexer 1026 capable of providing two signals for further opticalamplification using amplification elements 1030, 1032. Each opticalamplified signal as provided by the amplification elements 1030, 1032can be interfaced to an appropriate optical network 1034, 1036 fornetwork usage.

FIG. 11A shows an example of a system 1100 in which USPL-FSOtransceivers can be utilized for use in line-of-sight opticalcommunication (e.g. “lasercom”) applications, and FIG. 11B shows anexample of a system 1150 in which USPL-FSO transceivers can be utilizedfor use in non-line-of-sight lasercom applications. An advantage to someimplementations of the current subject matter can be realized due toscattering of the optical signal sent from a transmit element as thetransmitted light passes through the atmosphere. This scattering canpermit the use of non-line-of-sight communication. In addition, radiosused in such communication systems can operate in the solar-blindportion of the UV-C band, where light emits at a wavelength of 200 to280 nm. In this band, when solar radiation propagates through theenvironment, it is strongly attenuated by the Earth's atmosphere. Thismeans that, as it gets closer to the ground, the amount of backgroundnoise radiation drops dramatically, and low-power communications linkoperation is possible. On the other hand, environmental elements such asoxygen, ozone and water can weaken or interrupt the communicationsbroadcast, limiting usage to short-range applications.

When UV waves spread throughout the atmosphere, they are typicallystrongly scattered into a variety of signal paths. Signal scattering isessential to UV systems operating in non-line-of-sight conditions, andthe communications performance can highly dependent on the transmissionbeam pointing and the receiver's field of view. A line-of-sightarrangement 1100 as shown in FIG. 11A can differ in bandwidth size froma non-line-of-sight arrangement 1150 as shown in FIG. 11B. Ultravioletcommunication can more strongly rely on a transmitter's beam positionand a receiver's field of view. As a result, refining of the pointingapex angle, for example by experimenting with supplementary equipment toenhance the UV-C signal, can be advantageous.

FIG. 12 illustrates an example of a remote sensing system 1200 in whichan USPL source 102 is fiber coupled by an optical fiber component 202 toan optical launch element 1202 capable of transmitting and receivingoptical signals. Some of the light propagated forward including thelight from data signal through the optical launch element 1202 isbackscattered by interaction with air-borne particulates that are thesubject of investigation. The optical backscattered signal is detectedthrough the optical launch element 1202 or a similar receive apertureand passed along for detection and spectrographic analysis throughdetection circuitry 1204 or the like in FIG. 12 . The signature ofparticulates within a target atmospheric region 1206 within which aninvestigation is made can be calibrated through known approaches, forexample using predetermined spectrographic calibration measurementsbased on one or more of ultraviolet spectroscopy, infrared spectroscopy,Raman spectroscopy, etc. Consistent with this implementation, an opticalsystem can be operated as a LIDAR instrument providing enhancedresolution and detection sensitivity performance, using USPL lasersources operating over a spectral range of interest. Adjustability ofspectral range can aid in evaluating and analyzing chemical constituentsin the atmosphere.

USPL-FSO transceivers can be utilized for remote sensing and detectionfor signatures of airborne elements using ionization or non-ionizationdetection techniques, utilizing optical transport terminals manufacturedthrough either the Hyperbolic Mirror Fabrication Techniques orconventional Newtonian designs that focus a received signal at one idealpoint. Also certain adaptations can be related to ionization probing ofremote regions include controllable ionization, which has been shown tooccur at these frequencies and an ionization process, which can befocused at distance to adjust depth of atmospheric penetrationespecially in weather and clouds.

FIG. 13 illustrates an example of use of USPL sources as well as opticalreception techniques to improve detection sensitivity. Researchers atthe National Institute of Standards and Technology (NIST), US, havebuilt a laser ranging system that can pinpoint multiple objects withnanometer precision over distances up to 100 km. The LIDAR (lightdetection and ranging) system could have applications from precisionmanufacturing on Earth to maintaining networks of satellites in perfectformation (Nature Photonics DOI: 10.1038/NPHOTON.2009.94). The NISTdevice uses two coherent broadband fiber-laser frequency combs.Frequency combs output a series of stable short pulses that also containa highly coherent carrier that extends across the pulse train. Thismeans a frequency comb can be employed to simultaneously make aninterferometric measurement as well as a time-of-flight measurement,thereby enhancing analytical capabilities for application specificsituations.

In the arrangement shown in FIG. 13 , two phase-locked frequency combs1301 and 1302 are used in a coherent linear optical samplingconfiguration, also known as a multi-heterodyne, meaning that onefrequency comb measures both distance paths, while the other frequencycomb provides distance information encoded in the light of the firstcomb. Pulses from one frequency comb 1301 can be launched out of thefiber and directed towards two glass plates, a reference 1303 and atarget 1304. The plates 1303 and 1304 can reflect a certain fraction(e.g. approximately 4%) of the pulse back down the fiber, effectivelycreating two new pulses. The time separation between the two pulses 1301can give the distance between the moveable target plate and referenceplates. A second frequency comb 1302 is tightly phase-locked with thefirst, but has a slightly different repetition rate. Due to thedifferent delay between consecutive pulses when the sources interfere,the second frequency comb can sample a slightly different part of thelight from the electric field of the first comb.

Using the technique described is reference to FIG. 13 , it is possibleto replace the two coherent broadband fiber-laser sources with twoappropriate USPL sources used within the scope of the configurationoutlined having each USPL source fiber coupled to dedicated free-spaceoptical telescope designs. By doing so, the overall efficiency, opticalranging and accuracy can be improved substantially.

In some embodiments, a native pulse repetition rate of a USPL lasersource and may be 50 MHz or less, which may be undesirably low foroptical data transmission, limiting the system to low data rateapplications of 50 Mbps or less. Accordingly, systems to increase USPLoperational rates are needed for providing solutions for data transportin excess of 50 Mbps.

FIG. 14 illustrates an example of a remote sensing system 1400 in whichan USPL source 102 is fiber coupled by an optical fiber component 202 toan optical launch element 1202 capable of transmitting and receivingoptical signals. Light propagated forward by the optical launch element1202 including light from the data signal is backscattered byinteraction with targets known and unknown that are the subject ofinvestigation within an atmospheric region 1206. The opticalbackscattered signal including light from the data signal is detectedthrough the optical launch element 1202 or a similar receive apertureand passed along for detection analysis through a detection circuitryand spectrographic analysis component 1402 in FIG. 14 . The signature ofparticulates within the region 1206 under investigation can becalibrated, for example where range-finding analysis can be performed. Asystem 1400 as in FIG. 14 can include a USPL-FSO transceiver utilizedand operated across the infrared wavelength range as a range-finder andspotting apparatus for the purposes of target identification andinterrogation applications. As used herein, the term “optical” includesat least visible, infrared, and near-infrared wavelengths.

FIG. 15 illustrates an optical pulse multiplier module 1500 that canincrease the repetition rate of the output from a USPL source 102. Anexemplary USPL may have a pulse width of 10-100 femto-seconds and arepetition rate of, for example, 50 MHz. The output from the USPL 102can be fed as an input 1502 into a USPL photonic chip pulse multipliermodule 1504. In this example, the photonic chip can contain a 20,000:1splitter element 1506 that splits the input into discrete lightelements. Each light element on the opposite side of the splitterelement 1506 contains the 50 MHz pulse train. Each light element thenpasses through a delay controller (either a fiber loop or lens array)1510, which delays the pulse train for that element in time, for exampleby a number of picoseconds. Successive light elements are therebydelayed by incremental picoseconds. All of these pulse trains with theirunique time delays are combined into a single pulse train in a fashionsimilar to time division multiplexing utilizing a 20,000:1 opticalcombiner element 1512. The required ratios of splitters and combinerscan be controlled to provide necessary optical designs for theapplication required. The final output 1514 is a pulse train of 10-100femto-second pulses with a repletion rate of 1 THz. This THz pulse traincan then be modulated by a 10 or 100 GigE signal, such as shown in FIG.28 , resulting in 100 femto-second pulses per bit for the 10 GigEsystem, and 10 femto-second pulses per bit for 100 GigE systems. Theapplication cited is not limited to specific data rates of 10 and 100Gbps, but can operate as required by the application underconsiderations. These numbers are just for illustration purposes.Implementations of the current subject matter can use any multiplierfactor to increase the repetition rate of the USPL via the photonic chippulse multiplier module 1504 to any arbitrary repetition rate. Otherexamples used in generation of enhanced USPL repetition rates areillustrated within this submission.

FIG. 16 depicts a system 1600 for generation, transmission, andreceiving of high pulse rate USPL optical streams. An optical chipmultiplexing module 1610, which can for example be similar to thatdiscussed in reference to FIG. 15 , can be used in this application. Inthis approach to achieve USPL pulse multiplication, a series of 10 GigErouter connections (10 GigE is not intended to be a limiting feature)described by signals 1601, 1602, 1603, 1604 (four signals are shown inFIG. 16 , but it will be understood that any number is within the scopeof the current subject matter) are interfaced to the optical chipmultiplexing module 1610. In operation, the optical chip multiplexingmodule 1610 can support full duplex (Tx and Rx) to connect with the 10GigE routers 1601, 1602, 1603, 1604. The optical chip multiplexingmodule 1610 can provide efficient modulation by a USPL signal 1685output from a USPL source 1690 for ingress optical signals 1601, 1602,1603, 1604. The optical chip multiplexing module 1610 can providecapabilities to modulate and multiplex these ingress optical signals.

At a remote receive site where a receiving device is positioned, allsignals sent via a transmitting element 1660 at the transmitting devicecan be recovered using an appropriate receiver element 1665. Acomplementary set of optical chip multiplexing module 1675 can providenecessary capabilities for demultiplexing the received data stream asshown by elements for delivery to a series of routers 1601′, 1602′,1603′, 1604′ (again, the depiction of four such routers is not intendedto be limiting). End-to-end network connectivity can be demonstratedthrough network end-point elements.

FIG. 17 depicts an example system 1700 in which an optical chip isinterconnected to a wavelength division multiplexing (WDM) system. WDMsystems have the advantage of not requiring timing or synchronization asneeded with a 10 GigE (or other speed) router 1701, since each 10 GigEsignal runs independent of other such signals on its own wavelength.Timing or synchronization of the TDM optical chip with 10 GigE routerscan be important in a TDM optical chip. A GbE switch 1701 can providethe necessary electrical RF signal 1705, from the switch 1701 tomodulate a USPL source 1702, either directly or by use of USPL a pulsemultiplier module previously detailed within this document. A typical RZoutput 1710 can be coupled into an external modulator 1720, which can bemodulated using a NRZ clock source for the switch 1701, therebyresulting in a RZ modulated spectrum 1730. The conversion process usingreadily available equipment can provide capabilities for introducingUSPL sources and their benefits into the terrestrial backhaul networkspectrum.

For the optical chip system to successfully bridge between two remote 10GigE switches, the chip may act like a simple piece of fiber. The timingof the TDM chip can therefore be driven by the 10 GigE switch 1701. Bothactively mode-locked USPLs (i.e. 40 GHz, 1 picosecond pulse width) andpassively mode-locked USPLs (i.e. 50 MHz, 100 femtosecond pulse width)can be driven by a RF timing signal.

FIG. 18 illustrates a device 1800 that can support another approach toprogression to a high pulse repetition data rate operation, such as forextremely high data rate operation in which optical chip design can beperformed using either fiber or free-space optics. A 50 MHz USPL source1801 may be interfaced to a series of optical delay controller elements1802, which can be designed using either fiber loops or offset lenses,to result in producing exactly a 10.313 Gbps RZ output stream, which isthe 10 GigE line rate (greater than 10 Gbps because of 64B/66Bencoding). A splitter element 1803 provides splitting functionality ofthe incoming optical signal train 1801 into (in this example) 206 paths,along with variable optical delay lines 1804. After sufficient delay isintroduced through design all signals are multiplexed together through acombiner element 1805. In so doing a series of optical signals eachidentical, and equally spaced between adjacent pulses form a continuumof pulses for modulation. Prior to entering an E-O modulator element1806, all optical ingress signals can be conditioned by pre-emphasistechniques, for example using typical optical amplification techniques,to result in a uniform power spectrum for each egress signal from thecombiner element 1805. The conditioned egress signals may then becoupled into the E-O modulator element 1806 and modulated with anavailable NRZ signal from the 10 GigE signal source element 1807. The 10GigE modulated output 1809 can interface to an EDFA and then into the TXof a FSO system (or a fiber optic system). The Rx side (after thedetector) can be fed directly into a 10 GigE switch as a modulated andamplified output 1810.

FIG. 19 illustrates another example of a device 1900 that can be usedfor USPL pulse multiplication consistent with implementations of thecurrent subject matter. Consistent with this approach, a 10×TDM systemis configured to give a 100 Gbps output. A TDM demux chip can be on thereceive side of a communication link to break up the individual 10 GigEsignals, and can include a reciprocal approach to the design shown inFIG. 19 .

As in FIG. 18 , a 50 MHz USPL source 1801 may be interfaced to a seriesof optical delay controller elements 1802, which can be designed usingeither fiber loops or offset lenses, to result in producing exactly a10.313 Gbps RZ output stream, which is the 10 GigE line rate (greaterthan 10 Gbps because of 64B/66B encoding). A splitter element 1803provides splitting functionality of the incoming optical signal train1801 into (in this example) 206 paths, along with variable optical delaylines 1804. After sufficient delay is introduced through design allsignals are multiplexed together through a combiner element 1805.Instead of a single modulator element 1806 as shown in FIG. 18 ,however, the 10.313 GHz RZ output 1901 from the combiner element 1805may be fed into a second splitter element 1910, which in this case canbe a 10× splitter, which splits the optical signal into ten parallelpaths. Other implementations of this design can support various splitratios as required by design. Optical paths out from second splitterelement 1910 are individually connected to specified optical delay lines1920. Each individual delayed path is connected to a dedicated opticalmodulator of a set of optical modulators 1930 modulated with anavailable NRZ signal from the 10×10 GigE signal source element 1931,resulting in a series of modulated optical signals 1935. An opticalcombiner identified 1940 provides a single optical pulse train 1950. Theseries of optical pulses in the single optical pulse train 1950 can beinterfaced to an appropriate optical amplifier for desired opticalconditioning for network use.

FIG. 20 illustrates another example of a device 2000 that can be usedfor USPL pulse multiplication consistent with implementations of thecurrent subject matter. A device 2000 as depicted can provide theability to achieve high USPL pulse repetition data rates for networkapplications by modulation of the low repetition rate intra-channelpulses. By applying direct modulation of each channel on the delaycontroller, creation of a modulation scheme, which is not constrained bythe current speed limitations from the electronics technology, can bebeneficially accomplished. Implementations of the current subject mattercan provide a mechanism to enhance the data transmission capacity of asystem, by separately modulating individual channels at the currentstandard electronic modulation speed (in the example of FIG. 20 at therate of 100×10 GigE signal input 2001) and time-multiplexing thechannels into a single frequency high rep rate pulse stream. In thisapproach, the current standard, which is limited by the speed ofelectro-optic modulators (40 Gbps), can be enhanced by approximately Norders of magnitude, where N is the number of channels of thetime-multiplexer. For example, a 100 channel TDM with each channelamplitude modulated at the current standard data rate can be able tooffer data rates at speeds of up to 4 Tbs. N can be limited by the widthof the optical pulse itself. In the limit that information is carried 1bit/pulse, the time slot occupied by 1 bit is the width of the pulseitself (in that sense, RZ system would converge to a NRZ). For example,in the scheme, a 40 fs pulse width laser with a 40 GHz rep rate is ableto carry information at a maximum rate of 25 Tbps. This approach can beused in a 40 Gbps-channel modulation scheme (i.e., 1 bit every 25 ps)and can correspond to a capacity of N^(˜)625 channels in a singletransmission, which can be the number of 40 fs time intervals fitting ina 25 ps time interval. A significant advantage of this approach is theability to “optically enhance” an otherwise limited data capacitymodulation scheme, while still interfacing with the existing data ratelimited modulators. For example, an amplitude modulator based on aMach-Zehnder interferometer can be easily integrated in a TDM ICpackage, in that required is the ability to branch out the channel intotwo separate paths, add a tiny phase modulator (nonlinear crystal) inone of the paths, and combine the paths for interference.

FIG. 20 includes a USPL source 2010 coupled to a multi-port opticalsplitter element 2020. The number of optical ports identified need notbe limited to those described or shown herein. A series of optical delaylines 2030 provide required optical delays between each parallel pathfrom the multi-port optical splitter element 2020, and can be tailoredfor specific applications. The optical delay paths from the opticaldelay lines 2030 are summed together using an optical combiner element2035. The resulting combined optical data stream appearing throughelement 2040 represents a multiplicative enhancement in the pulserepetition rate of the original USPL source identified by element 2010.Further enhancement in pulse repetition rate is accomplished though theusage of element 2041, described by an optical splitter where theincoming signal 2040 is split into a series of paths not limited tothose identified by element 2041. By way of a second delay controller2045, optical delays may be introduced to each path within the device asidentified by the second set of optical delay paths 2042. Each parallelpath 2042 in turn is modulated by a modulating element 2044 with anavailable RF signal source element identified by the signal input 2001.An optical combiner element 2050 integrates all incoming signals onto asingle data stream 2060.

Optical pre-emphasis and de-emphasis techniques can be introduced withineach segment of elements described to custom tailor the optical spectrumfor a uniform or asymmetric optical power distribution. Pre- &de-emphasis can be accomplished using commonly used optical amplifierssuch as Er-Doped Optical amplifiers (EDFA).

FIG. 21 depicts an example of a system 2100 that includes a mode-lockedUSPL source 2101, which can be used to generate appropriately requiredclock and data streams for the application. Mode-locked lasers canrepresent a choice of high performance, high finesse source for clocksin digital communication systems. In this respect, mode-locked fiberlasers—in either linear or ring configuration—can make an attractivecandidate of choice, as they can achieve pulse widths on the USPL sourceregion and repetition rate as high as GHz. In addition to that, fibersoffer compactness, low cost, low sensitivity to thermal noise, lowjitter, no problems associated with diffraction or air dust pollution,just to name a few. In a communications scenario, the pulse width candetermine the available bandwidth of the system, and the repetition ratelimits the data rate. The pulse width can be determined by the intrinsiccharacteristics of the laser cavity—i.e. balancing of the overallgroup-velocity dispersion (GVD), and the choice of the saturableabsorber (in the case of a passive system)—or the bandwidth of an activeelement (in the case of an active mode-locked system). The repetitionrate of the pulse train is constrained by the length of the fiber. Forexample, in a linear laser, the fundamental mode vos, of the laser canbe expressed as:v _(osc) =c/2n _(g) Lwhere c is the speed of light in vacuum, n_(g) is the average groupindex, and L is the length of the cavity. Therefore, a 10 cm long fiberlaser cavity element 2110 with an average group index of 1.47 would havea repetition rate of 1 GHz. In strictly passive systems, mode-lockingcan be achieved through the use of a saturable absorber. In an activelaser, an amplitude modulator element 2150 can be inserted in the cavityto increase the repetition rate of the laser (harmonic mode locking). Inorder to achieve high repetition rate clocks using mode-locked USPLsource, it is possible to use one or more of (i) an intra-cavityamplitude Mach-Zehnder modulator (MZM) 2150 as shown in FIG. 21 and (ii)a low threshold saturable absorber. These techniques, known as “harmonicmode-locking”, can be utilized within a fiber based plant distributionsystem or within a FSO system, for terrestrial, submarine or FSO systemeither in air, space or submarine applications.

Detailed within FIG. 21 is 980 nm pump element 2102 coupled to anoptical WDM device 2105. An erbium doped optical amplifier 2110 orequivalent can be used to create a non-linear environment to obtain amode-locked pulse train emission within a closed cavity establishedbetween two Faraday reflectors 2101 and 2160 on either end of theoptical USPL cavity. Operation of the device is capable of establishinga self-contained series of optical pulse in excess of 100 Gbps, andhighly synchronized in nature at the output port 2170 of the module. Inorder to achieve a high gain non-linear medium the EDFA 2110 can bespecially designed. A phase lock loop 2130 can provide advantageousstability in operation by maintaining a synchronized clock sourcethrough modulation of the signal through components 2120, 2130, 2150 ofthe self-contained high-repetition rate pulse generator. To achieve highrep rates in a laser that is limited by its dimensions (length in thecase of a linear laser and perimeter in the case of a ring laser), itcan be necessary to stimulate intra-cavity generation of the multiplesof the fundamental mode. In the active case, an amplitude modulatorinserted in the cavity modulates the loss of the system operating as a“threshold gating” device. For this approach to be successful, it can benecessary that the controlling signal to the modulator be referenced tothe oscillation of the laser itself to avoid the driving signal“forcing” an external frequency of oscillation on the laser. This can berealized by the introduction of a phase-lock-loop element 2130, or asynchronous oscillator circuit to track-and-lock onto the repetitionrate of the laser, and regenerate the signal. In the case of a PLL, theRF output can be set to a multiple of the input signal (much as thisdevice is used in cell phone technology), and the rep rate of the laserincreased. The signal can then be used for triggering of a pulsegenerator, or in conjunction with a low-pass filter. A MZ amplitudemodulator 2150 outside the laser cavity can be used to create On-OffKeying (OOK) modulation on the pulse train coming out of the mode-lockedlaser.

FIG. 22 shows a graphical depiction 2200 illustrating effects of a lossmodulation introduced to the input pulse train 2201 due to the presenceof the amplitude modulator 2205 with a controlling signal NRZ signal2210 made of a bit sequence as illustrated. The resulting signal at theoutput of the device 2220 represents an NRZ to RZ converter device foruse in telecommunications and scientific applications where theapplication may benefit from RZ data streams. A clock signal 2201(optical input) at a given pulse repetition rate will pass through themodulator 2205. At the same time, a controlling signal consisting of asequence of 1's and 0's can be applied to the RF port of the modulatorelement 2215. When the modulator element 2215 is biased at minimumtransmission, in the absence of a controlling signal the lossexperienced by the optical signal can be at its maximum. In the presenceof the RF signal (1's), the loss will drop to a minimum (OPEN GATE),thus working as an On-Off Keying modulation device. The pulse width ofthe output optical signal is typically much less than the time slotoccupied by a single bit of information (even less than a half clockperiod of a NRZ scheme) making this system genuinely RZ as identified byelement 2220.

FIG. 23 illustrates an example system 2300 for generation of highoptical harmonic USPL pulse streams having high pulse repetition rateusing a saturable absorber (SA) device 2330. The SA device 2330 can insome examples include carbon nanotubes. Passive mode-locked fiber lasersusing carbon nanotubes SA (CNT-SA) make another attractive option forhigh rep rate sources due to their ability to generate high harmonics ofthe fundamental rep rate. In the approach described, a closed,self-contained optical cavity is established, in which two Faradayreflectors 2301 and 2350 form the optical cavity. Although a high-powererbium doped fiber amplifier (EDFA) 2310 is shown in FIG. 23 , anyinverting medium producing a non-linear optical cavity can be used. Aseed laser 2315, such as for example a 980 nm pump laser as shown inFIG. 23 can be used in generating a high-repetition rate optical train.In particular, any suitable pump laser may be considered in terms ofoptical wavelength and pulse repetition rate required. The SA element2330 can be placed within the cavity to establish required optical pulsecharacteristics 2350 as required through design requirements.

FIG. 23 shows the schematics of an example of a laser that can be usedin one or more implementations of the current subject matter. Unlike theactive laser shown in FIG. 22 , here the MZ modulator can be replaced bythe SA element 2330. A technique similar to those described herein canbe utilized within a fiber based plant distribution system or within aFSO system, for terrestrial, submarine or FSO system either in air,space or submarine applications.

FIG. 24 illustrates an approach to providing time-domain multiplexing(TDM) where the TDM multiplexes a pulse train using parallel time delaychannels. In some instances, it can become important to manipulate thedelay channels such that they are “consistent” relative to each another.The frequency of the output multiplexed pulse train can ideally as muchas possible be insensitive to environmental changes. For that, aproposed feedback loop control system is design to correct the delayunits for any fluctuations which compromises the stability of the outputrep rate.

FIG. 24 shows a diagram of an example of a delay control system 2400.The control loop can be implemented in one of several ways consistentwith the current subject matter. FIG. 24 describes one possibility forillustration purposes. The input pulse train enters the TDM andmultiplexes into N paths, each with its own delay line. If the paths aremade of low “bending-loss” fiber waveguides, then each path can becoiled around a cylindrical piezoelectric actuator (PZ) of radius R. Theactuators generally expand in a radial direction as a result of acontrolling voltage (Vc). This expansion ΔR, which is linearlyproportional to Vc, causes a change in length of the fiber ΔL=2πNΔR,where N is the number of fiber turns around the PZ. For Terahertzmultiplexing, the delay between the pulses (and thus of PZ1) must be 1picosecond. This can require a change in length equals to 200 microns,which, for a one turn PZ actuator corresponds to a ΔR=32.5 microns. Mostcommercially available piezoelectric actuators are highly linear andoperate well within this range. The control mechanism can, therefore, bebased on several PZ actuators, each with a number of turns correspondingto multiples of the first delay, i.e. (32, 64, 96 microns, etc.), andcontrolled by a single voltage Vc. The controlling voltage is determinedby the feedback system, which compares the frequency of the outputsignal using a 1/N divider, with the frequency of the input signal,using a phase comparator (PC). The frequency of the “slow” input opticalsignal (represented by the waveform with TRT in FIG. 24 is converted toan RF signal using photo-detector PDin. In order to reduce the effectsof electronic jitter, a “differentiator” (or high pass filter) can beapplied to the RF signal as to steepen the leading edges of the pulses.A phase-locked loop is used to track-and-lock the signal, and toregenerate it into a 50% duty-cycle waveform. Likewise, in the outputside, the optical signal is picked-up by photo-detector PDout, high-passfiltered, and regenerated using the clock output port of aclock-and-data recovery system. The clock of the output signal, whichhas a frequency N times the frequency of the input signal, is send to anN times frequency divider before going to the phase comparator. From thephase comparator, a DC voltage level representing the mismatch betweenthe input and output signals (much as what is used in the architectureof PLL circuits) indicates the direction of correction for theactuators. A low-pass filter adds a time constant to the system toenhance its insensitivity to spurious noise.

A CDR can advantageously be used in the output, as opposed to a PLL suchthat the output signal may, or may not, be modulated. This system can bedesigned to work in both un-modulated, and “intra-TDM modulated” (i.e.one modulator at each delay path) schemes. However, this is a completelydeterministic approach to compensating for variations on the length ofthe delay lines. Ideally, and within a practical standpoint, the delaypaths should all be referenced to the same “thermal level” i.e. besensitive to the same thermal changes simultaneously. In the event thateach line senses different variation, this system would not be able tocorrect for that in real time.

In the alternative, a completely statistical approach can includesumming of op amp circuits (S1 . . . SN) to deliver the controllingvoltage to the actuators. Using such an approach, input voltages (V1 toVN) can be used to compensate for discrepancies in length between thelines, in a completely static sense, otherwise they can be used forinitial fine adjustments to the system. The approach typically must alsocompensate or at least take into account any bending loss requirementsof the fibers used. Some new fibers just coming out in the market mayhave a critical radius of only a few millimeters.

In the event that each path delay line senses different variation intemperature or experiences uncorrelated length changes due to spuriouslocalized noise, the previously described approach, as is, may sufferfrom difficulties in performing a real time correction. A more robustapproach operating in a completely statistical sense can be usedconsistent with some implementations of the current subject matter. Insuch an approach, summing op amp circuits (S1 . . . SN) can be used todeliver the controlling voltages to the actuators. In this case, theinput voltages (V1 to VN) can be used to compensate for discrepancies inlength between the delay lines in a completely statistical sense,otherwise they can only be useful for initial fine adjustments to thesystem (calibration).

Referring again to FIG. 24 , an incoming USPL source identified aselement 2401 is coupled to an optical coupler element 2403, such thatone leg of the coupler connects to an optical photodiode selected foroperation at the operational data rate of 2401. Using standardelectronic filtering techniques described by elements 2404, 2405, and2406 an electrical square wave representation of the incoming USPLsignal is extracted and identified by element 2407. The second opticalleg of coupler 2403 is interfaced into an appropriate optical splitterelement identified by 2410, where the incoming signal into 2410 is splitinto 206 parallel optical paths. Also illustrated are variable rateoptical delay lines established in parallel for each of the parallelbranches of the splitter element 2410. The parallel piezoelectricelements are identified by elements 242N and are controlledelectronically through feedback circuitry within the diagram. A controlvoltage identified by Vc is generated through a photodiode 2485 alongwith electronic circuitry elements 2480 and 2475. The clock-and-dataRecovery (CDR) element 2475 produces a clock source that is used incontrolling each of the PZ elements. Optical paths identified as 244Nare combined after a proper delay is introduced into each leg of element2410. The pulse multiplied USPL signal 2490 is thereby generated.

FIG. 25A shows a schematic of a fiber PZ actuator 2500, and FIG. 25Bshows a graph 2590 of radius vs. voltage for such an actuator. Together,these drawings illustrate operation of a PZ actuator for increasing thepulse repetition rate of an incoming USPL pulse train through inducedoptical delay. Although shown for use as an element for enhancing pulserepetition rate generation for USPL signals, the same technique can beused for other optical devices requiring or benefiting from opticaldelay. The basic structure for the device is a fiber based PZ actuator2501. When a voltage 2550 is applied to electrodes 2520 a voltageinduced stress results within the fiber, causing a time delay of theoptical signal traveling through the fiber. By varying applied voltage aperformance curve of optical delay vs. applied voltage is obtained asshown in the graph 2590 of FIG. 25B.

FIG. 26 shows a diagram illustrating features of an example statisticalcorrector 2600. The coarse correction controller 2640 shown in FIG. 26corresponds to the system described in the previous section, which cancorrect for length variations simultaneously picked up by all delaylines. As mentioned, these variations are expected to occur in a timescale much slower than the “intra delay line” spurious variations. Thislatter effect can manifest itself as a period-to-period jitterintroduced on the system. This type of jitter can be monitored using anRF Spectrum Analyzer (RFA), causing the rep rate line of the system todisplay “side lines” (or side bands), which are the result of theanalyzer beating together noisy frequencies resulting from uneven timeintervals between consecutive pulses. One such pattern can be processedusing an analog-to-digital converter (ADC) and saved as an array ofvalues, which can then be fed to a neural network (NN) machine. Neuralnetwork machines are known to possess excellent adaptabilitycharacteristics that allow them to essentially learn patterns fromoutside events by adapting to new set of input and outputs. A set ofinputs in this case can be generated from a set of “imperfectobservations”, i.e. “noisy” outputs of the TDM system as detected by theRFA and converted to digital arrays by the ADC ({f1, f2, . . . , fN},where fi is a frequency component picked up by the RFA). A set ofoutputs can be generated from the corrections ({V1, V2, . . . , VN},where Vt is a compensating input voltage to the summing op amp) requiredto rid the output frequency set from the undesired excess frequencynoise, which is due to the outside perturbations to the system. With asufficiently large number of {f,V} pairs, where f, V are frequency,voltage arrays, one can build an statistical set to train the NN machineto learn the underlying pattern associated with the presence of theintra-channel noise. These machines can be found commercially in an ICformat from several manufacturers, or implemented as software and usedin conjunction with a computer feedback control mechanism. A singlelayer Perceptron type neural network, or ADALINE (Adaptive Linear Neuronor later Adaptive Linear Element), should be sufficient to accomplishthe task.

Similar to the description provided above in relation to FIG. 24 , astatistical corrector element 2670 can include electronic circuitry thatis similar to or that provides similar functionality as the electricalcircuitry elements 2480 and 2475 and the photodiode 2485 of FIG. 24 .For the approach illustrated in FIG. 26 , a RF spectra analyzer 2695along with a Neural Network 2670 and a Coarse Correction Controllerelement 2640 are used to perform the requirement of optical delayintroduced into a parallel series of PZ elements 262N.

FIG. 27 illustrates concepts and capabilities of approaches consistentwith implementations of the current subject matter in which performance,accuracy, and resolution can be improved through replacement ofpiezoelectric disk (PZ) modules identified by elements 2795 and 272N,where compact micro fiber based collimators (MFC) 2795 encircled byceramic disks are used to obtain optical delay lines. Althoughillustrating a technique for increasing the native pulse repetition ratefor a USPL pulse train, the design illustrated is not limited to suchapplications but can be applied or extended to other needs within theoptical sector wherever optical delay is required. In so doing, a morecontrolled amount of temporal delay can be introduced within each MFCelement of the circuit. The improvement through the use of utilizing MFCelements can improve response, resolution, and the achievement ofreproducing in a rapid fashion required voltage responses in a massproduction means. The concept identified within FIG. 27 can beincorporated into precisely produced elements that can serve ascomplementary paired units for use in reducing USPL pulse-to-pulsejitter as well as for the purposes of data encryption needs.

With further reference to FIG. 27 , a USPL source 2701 having a certainpulse repetition rate is split into a preselected number of opticalpaths 271N (which can number other than 206) as identified by splitterelement 2705. An appropriately controlled delay 273N is introduced intoeach parallel leg of the split optical paths 271N using elementsdescribed by 2795 and 272N. The resulting delayed paths 274N are addedtogether through an optical combiner element 2760. The pulse multipliedUSPL signal 2780 results.

One potential disadvantage of some previously available TDM designs, inwhich fibers are “wrapped-around” the piezo actuators, is that themechanism must comply with the bending loss requirements of the fibersused. Some new fibers just coming out in the market have critical radiusof only a few millimeters. To correct for this issue, implementations ofthe current subject matter can use of micro-machined air-gap U-bracketsin lieu of the fiber-wrapped cylindrical piezo elements. FIG. 27illustrates this principle. In this approach, the piezoelectricactuators (PZ1, . . . PZN) can be replaced by air gap U-bracketstructures constructed using micro-fiber collimators (MFCs), andmicro-rings made of a piezoelectric material. In this case, however, thepiezoelectric actuator expands longitudinally, increasing (ordecreasing) the air gap distance between the collimators, in response tothe controlling voltages (V1, V2, . . . VN). As in the case of thecylindrical piezoelectric, a single voltage Vc can be use to drive allthe piezoelectric devices, provided that the gains of each channel (G1,G2, . . . GN) are adjusted accordingly to provide the correct expansionfor each line. Ideally, except for inherent biases to the system (i.e.intrinsic differences between op amps), the gain adjustments should beas G1, 2G1, 3G1, and so forth, in order to provide expansions, which aremultiples of the TRT/N. Another way of implementing such an approach canbe the use of multiple piezoelectric rings at the channels. In thatmanner, one can have channels with 1, 2, 3, N piezoelectric rings drivenby the same voltage with all amplifiers at the same gain.

FIG. 28 provides a conceptual presentation of an optical chip system2800 to successfully bridge between two remote 10 GigE switches.Ideally, such a connection can perform similarly to a simple piece offiber. The timing of the TDM chip can be driven by the 10 GigE switch.

In reference to FIG. 28 , a USPL source 2805 having a predeterminednative pulse repetition rate identified by 2806 connects to an opticalPulse multiplier chip 2807. Element 2807 is designed to convert theincoming pulse repetition rate signal 2806 into an appropriate level foroperation with high-speed network Ethernet switches as identified by2801. Switch 2801 provides a reference signal 2802 used to modulatesignal 2809 by way of a standard electro-optic modulator 2820 at thedata rate of interest. A resulting RZ optical signal is generated asshown in element 2840.

An alternative to having the timing run from the 10 GigE switch is tobuildup the USPL to a Terabit/second (or faster) with a multiplierphotonic chip, and then modulate this Terabit/second signal directlyfrom the 10 GigE switch. Each bit will have 100 or so pulses. Anadvantage of this approach can be the elimination of a need for separatetiming signals to be run from the switch to the USPL. The USPL viamultiplier chip just has to pump out the Terabit/second pulses. Anotheradvantage is that the output of the Multiplier Chip does not have to beexactly 10.313 or 103.12 Gbps. It just has to at a rate at about 1Terabit/second. Where each 10 GigE bit has 100 or 101 or 99 pulses, thislimitation is a non-issue. Another advantage is each bit will have many10 USPL, so the 10 GigE signal will have the atmospheric propagation(fog and scintillation) advantage. Another advantage can be realized atthe receiver end. It should be easier for a detector to detect a bit ifthat bit has 100 or so USPL pulses within that single bit. This couldresult in improved receiver sensitivity, and thus allow improved rangefor the FSO system. An additional advantage can be realized in thatupgrading to 100 GigE can be as simple as replacing the 10 GigE switchwith a 100 GigE switch. Each bit will have around 10 pulses in thiscase.

From a purely signal processing perspective this approach demonstratesan efficient way to send data and clock combined in a singletransmission stream. Much like a “sampling” of the bits using an opticalpulse stream, this approach has the advantage that the bit “size” isdetermined by the maximum number of pulses the it carries, thereforeestablishing a basis for counting bits as they arrive at the receivingend. In other words, if the bit unit has a time slot which can fit Npulses, the clock of the system can be established as “one new bit ofinformation” after every 5th.

A technique similar to those described herein can be utilized within afiber based plant distribution system or within a FSO system, forterrestrial, submarine or FSO system either in air, space or submarineapplications, and illustrates for the first time how the interconnectionfrom USPL sources to optical network elements is achieved for networkingapplications.

FIG. 29 shows a system 2900 that illustrates a conceptual networkextension for the design concept reflected within FIG. 28 . As multipleUSPL sources 2901, 2902, 2903 (it should be noted that while three areshown, any number is within the scope of the current subject matter),each modulated through dedicated optical switches and USPL laserMultiplier Chips circuits are configured in a WDM arrangement. Asdescribed in reference to FIG. 28 , electrical signals from eachEthernet switch can be used to modulate dedicated optical modulators2911, 2922, 2928 for each optical path. Optical power for each segmentof the system can be provided by optical amplification elements 2931,2932, 2933 for amplification purposes. Each amplified USPL path can thenbe interfaced to an appropriate optical combiner 2940 for transport to anetwork 2950, and can be either free space or fiber based as required.The output from the WDM module can then be configured to a transmittingelement 102 for FSO transport or into fiber plant equipment.

The technique described herein can be utilized within a fiber basedplant distribution system or within a FSO system, for terrestrial,submarine or FSO system either in; air, space or submarine applications,and illustrates for the first time how the interconnection from USPLsources to optical network elements is achieved for networkingapplications.

FIG. 30 shows the schematics of an experimental setup forimplementations of the current subject matter to include construction ofa computer assisted system to control the pulse width of an all-fibermode-locked laser using recursive linear polarization adjustments withsimultaneous stabilization of the cavity's repetition rate using asynchronous self-regenerative mechanism. The design can also offertune-ability of the repetition rate, and pulse width.

The fiber ring laser is represented by the inner blue loop, where allintra-cavity fiber branches are coded in blue, except for the positivehigh dispersion fiber outside the loop, which is part of the fibergrating compressor (coded in dark brown). The outside loops representthe feedback active systems.

FIG. 30 shows a diagram of a system 3000 illustrating features of anUSPL module providing control of pulse width and pulse repetition ratecontrol through mirrors (M1, M2), gratings (G1,G2), lengths (L1,L2),second-harmonic generator (SHG), photomultiplier tube (PMT), lock-inamplifier (LIA), data acquisition system (DAC), detector (DET),clock-extraction mechanism (CLK), frequency-to-voltage controller (FVC),high-voltage driver (HVD), reference signal (REF), pulse-generator(PGEN), amplitude modulator (AM), isolator (ISO), piezoelectric actuator(PZT), optical coupler (OC), polarizer (POL), and polarizationcontroller (PC) all serve to provide control of pulse repetition rateand pulse width control.

The passive mode-locking mechanism can be based on nonlinearpolarization rotation (NPR), which can be used in mode-locked fiberlasers. In this mechanism, weakly birefringent single mode fibers (SMF)can be used to create elliptically polarized light in a propagatingpulse. As the pulse travels along the fiber, it experiences a nonlineareffect, where an intensity dependent polarization rotation occurs. Bythe time the pulse reaches the polarization controller (PC) 3001 thepolarization state of the high intensity portion of the pulseexperiences more rotation than the lower intensity one. The controllercan perform the function of rotating the high intensity polarizationcomponent of the pulse, bringing its orientation as nearly aligned tothe axis of the polarizer (POL) as possible. Consequently, as the pulsepasses through the polarizer, its lower intensity components experiencemore attenuation than the high intensity components. The pulse comingout of the polarizer is, therefore, narrowed, and the entire processworks as a Fast-Saturable Absorber (FSA). This nonlinear effect works inconjunction with the Group-Velocity Dispersion (GVD) of the loop, and,after a number of round trips, a situation of stability occurs, andpassive mode-locking is achieved. The overall GVD of the optical loopcan be tailored to produce, within a margin of error, an specificdesired pulse width, by using different types of fibers (such as singlemode, dispersion shifted, polarization maintaining, etc. . . . ), andadding up their contributions to the average GVD of the laser.

An active control of the linear polarization rotation from the PC cangreatly improve the performance of the laser. This can be achieved usinga feedback system that tracks down the evolution of the pulse width.This system, represented by the outer loop in FIG. 1 , can be used tomaximize compression, and consequently, the average power of the pulse.A pulse coming out of the fiber ring laser through an OC is expected tohave a width on the order of a few picoseconds. An external pulsecompression scheme, which uses a fiber grating compressor, is used tonarrow the pulse to a sub 100 fsec range. This technique has beenextensively used in many reported experiments, leading to high energy,high power, USPL pulses. Here, the narrowed pulse is focused on aSecond-Harmonic Generator (SHG) crystal and detected using aPhoto-Multiplying Tube (PMT). The lock-in-amplifier (LIA) provides anoutput DC signal to a Data Acquisition Card (DAC). This signal followsvariations of the pulse width by tracking increases, or decreases, inthe pulses' peak power. A similar technique has been successfully usedin the past, except that, in that case, a Spatial Light Modulator (SLM)was used instead. Here, a programmable servo-mechanism directly controlsthe linear polarization rotation using actuators on the PC. With the DCsignal data provided by the DAC, a decision-making software (such as,but not limited to, LABVIEW or MATLAB SIMULINK) can be developed tocontrol the servo-mechanism, which in turn adjusts the angle of rotationof the input pulse relative to the polarizer's axis. These adjustments,performed by the actuators, are achieved using stress inducedbirefringence. For instance, if the pulse width decreases, the mechanismwill prompt the actuator to follow a certain direction of the linearangular rotation to compensate for that, and if the pulse widthincreases, it will act in the opposite direction, both aimed atmaximizing the average output power.

A self-regenerative feedback system synchronized to the repetition rateof the optical oscillation, and used as a driving signal to an amplitudemodulator (AM), can regulate the round trip time of the laser. In theactive system, the amplitude modulator acts as a threshold gating deviceby modulating the loss, synchronously with the round trip time. Thistechnique has can successfully stabilize mode-locked lasers in recentreports. A signal picked up from an optical coupler (OC) by aphoto-detector (DET) can be electronically locked and regenerated by aclock extraction mechanism (CLK) such as a Phase-Locked Loop or aSynchronous Oscillator. The regenerated signal triggers a PulseGenerator (PGen), which is then used to drive the modulator. In aperfectly synchronized scenario, the AM will “open” every time the pulsepasses through it, at each round trip time (TRT). Because the CLKfollows variations on TRT, the driving signal of the AM will varyaccordingly.

An outside reference signal (REF) can be used to tune the repetitionrate of the cavity. It can be compared to the recovered signal from theCLK using a mixer, and the output used to drive a Piezoelectric (PZT)system, which can regulate the length of the cavity. Such use of a PZTsystem to regulate the cavity's length is a well-known concept, andsimilar designs have already been successfully demonstratedexperimentally. Here a linear Frequency-to-Voltage Converter (FVC) maybe calibrated to provide an input signal to the PZT's High VoltageDriver (HVD). The PZT will adjust the length of the cavity to match therepetition rate of the REF signal. If, for instance the REF signalincreases its frequency, the output of the FVC will decrease, and sowill the HV drive level to the piezoelectric-cylinder, forcing it tocontract and, consequently increasing the repetition rate of the laser.The opposite occurs when the rep. rate of the reference decreases.

It is possible to have the width of the pulse tuned to a“transformed-limited” value using a pair of negative dispersiongratings. This chirped pulse compression technique is well established,and there has been reports of pulse compressions as narrow as 6 fs. Theidea is to have the grating pair pulse compressor mounted on a movingstage that translates along a line which sets the separation between thegratings. As the distance changes, so does the compression factor.

In an example of a data modulation scheme consistent withimplementations of the current subject matter, a passively mode lockedlaser can be used as the source of ultrafast pulses, which limits ourflexibility to change the data modulation rate. In order to scale up thedata rate of our system, we need to increase the base repetition rate ofour pulse source. Traditionally, the repetition rate of a passively modelocked laser has been increased by either shortening the laser cavitylength or by harmonic mode-locking of the laser. Both techniques causethe intra-cavity pulse peak power to decrease, resulting in longerpulse-widths and more unstable mode-locking.

One approach to solving this problem involves use of a modified pulseinterleaving scheme, by a technique which we call pulse multiplication.FIG. 31 illustrates this concept. The lower repetition rate pulse trainof a well-characterized, well-mode locked laser 3101 is coupled into anintegrated-optical directional coupler 3180, where a well-determinedfraction of the pulse is tapped off and “re-circulated” in an opticalloop with an optical delay 3150 equal to the desired inter-pulse spacingin the output pulse train, and re-coupled to the output of thedirectional coupler. For instance, to generate a 1 GHz pulse train froma 10 MHz pulse train, an optical delay of Ins is required, and to enablethe 100th pulse in the train to coincide with the input pulse from the10 MHz source, the optical delay might have to be precisely controlled.The optical delay loop includes optical gain 3120 to compensate forsignal attenuation, dispersion compensation 3160 to restore pulse-widthand active optical delay control 3150. Once the pulse multiplication hasoccurred, the output pulse train is OOK-modulated 3175 with a datastream 3182 to generated RZ signal 3190, and amplified in anerbium-doped fiber amplifier 3185 to bring the pulse energy up to thesame level as that of the input pulse train (or up to the desired outputpulse energy level).

One or more of the features described herein, whether taken alone or incombination, can be included in various aspects or implementations ofthe current subject matter. For example, in some aspects, an opticalwireless communication system can include at least one USPL lasersource, which can optionally include one or more of pico-second,nano-second, femto-second and atto-second type laser sources. An opticalwireless communication system can include USPL sources that can befiber-coupled or free-space coupled to an optical transport system, canbe modulated using one or more modulation techniques forpoint-to-multi-point communications system architectures, and/or canutilize optical transport terminals or telescopes manufactured throughone or more of hyperbolic mirror fabrication techniques, conventionalNewtonian mirror fabrication techniques, or other techniques that arefunctionally equivalent or similar. Aspheric optical designs can also oralternatively be used to minimize, reduce, etc. obscuration of areceived optical signal.

Free-space optical transport systems consistent with implementations ofthe current subject matter can utilize USPL laser designs that focus areceived signal at one ideal point. In some implementations onetelescope or other optical element for focusing and delivering light canbe considered as a transmitting element and a second telescope or otheroptical element for focusing and receiving light positioned remotelyfrom the first telescope or other optical element can function as areceiving element to create an optical data-link. Both opticalcommunication platforms can optionally include components necessary toprovide both transmit and receive functions, and can be referred to asUSPL optical transceivers. Either or both of the telescopes or otheroptical elements for focusing and delivering light can be coupled to atransmitting USPL source through either via optical fiber or by afree-space coupling to the transmitting element. Either or both of thetelescopes or other optical elements for focusing and receiving lightcan be coupled to a receive endpoint through either optical fiber or afree-space coupling to the optical receiver. A free-space optical (FSO)wireless communication system including one or more USPL sources can beused: within the framework of an optical communications network, inconjunction with the fiber-optic backhaul network (and can be usedtransparently within optical communications networks within an opticalcommunications network (and can be modulated using On-Off keying (OOK)Non-Return-to-Zero (NRZ), and Return-to-Zero (RZ) modulation techniques,within the 1550 nm optical communications band), within an opticalcommunications network (and can be modulated usingDifferential-Phase-Shift Keying (DPSK) modulation techniques), within anoptical communications network (and can be modulated using commonly usedmodulation techniques for point-to-point communications systemarchitectures using commonly used free-space optical transceiverterminals), within an optical communications network utilizing D-TEKdetection techniques, within a communications network for use inconjunction with Erbium-Doped Fiber Amplifiers (EDFA) as well as highpower Erbium-Ytterbium Doped Fiber Amplifiers (Er/Yb-DFA), within anoptical communications network (and can be modulated using commonly usedmodulation techniques for point-to-multi-point communications systemarchitectures), etc.

USPL technology can, in some aspects, be utilized as a beacon source toproviding optical tracking and beam steering for use in auto-trackingcapabilities and for maintaining terminal co-alignment during operation.The recovered clock and data extracted at the receive terminal can beused for multi-hop spans for use in extending network reach. The opticalnetwork can be provided with similar benefits in WDM configurations,thereby increasing the magnitude of effective optical bandwidth of thecarrier data link. USP laser sources can also or alternatively bepolarization multiplexed onto the transmitted optical signal to providepolarization multiplex USP-FSO (PM-USP-FSO) functionality. The recoveredclock and data extracted at the receive terminal can be used formulti-hop spans for use in extending network reach, and can include ageneric, large bandwidth range of operation for providing data-rateinvariant operation. An optical pre-amplifier or semi-conductor opticalamplifier (SOA) can be used prior to the optical receiver element and,alternatively or in combination with the recovered clock and dataextracted at the receive terminal, can be used for multi-hop spans foruse in extending network reach, having a generic, large bandwidth rangeof operation for providing data-rate invariant operation. Terminalco-alignment can be maintained during operation, such that significantimprovement in performance and terminal co-alignment can be realizedthrough the use of USPL technology, through the use of USPL data sourceas well as providing a improved approach to maintaining transceiveralignment through the use of USPL laser beacons.

USPL-FSO transceivers can be utilized in some aspects for performingremote-sensing and detection for signatures of airborne elements usingionization or non-ionization detection techniques, utilizing opticaltransport terminals manufactured through either the Hyperbolic MirrorFabrication Techniques or conventional Newtonian designs that focus areceived signal at one ideal point. USPL-FSO transceivers consistentwith implementations of the current subject matter can be utilized innon-line of sight lasercom applications. USPL-FSO transceiversconsistent with implementations of the current subject matter can allowadjustment of the distance at which the scattering effect (enabling NLOStechnique) takes place, reception techniques to improve detectionsensitivity using DTech detection schemes, and improved bandwidth viabroadband detectors including frequency combs. USPL-FSO transceiversconsistent with implementations of the current subject matter can beutilized in conjunction with Adaptive Optic (AO) Techniques forperforming incoming optical wave-front correction (AO-USPL-FSO).USPL-FSO transceivers consistent with implementations of the currentsubject matter can be utilized and operate across the infraredwavelength range. USPL-FSO transceivers consistent with implementationsof the current subject matter can be utilized in conjunction withoptical add-drop and optical multiplexing techniques, in bothsingle-mode as well as multi-mode fiber configurations. A USPL-FSOtransceiver consistent with implementations of the current subjectmatter can be utilized and operated across the infrared wavelength rangeas a range-finder and spotting apparatus for the purposes of targetidentification and interrogation applications.

In other aspects of the current subject matter, a series of switchednetwork connections, such as for example 10 GigE, 100 GigE, or the likeconnections can be connected from one point to another, either overfiber or free-space optics, for example via Time Division Multiplexing(TDM).

A mode-locked USPL source consistent with implementations of the currentsubject matter can be used to generate both clock and data streams.Mode-locked lasers can represent a choice of a high performance, highfinesse source for clocks in digital communication systems. In thisrespect, mode-locked fiber lasers—in either linear or ringconfiguration—can make an attractive candidate of choice, as they canachieve pulse widths of the USPL sources region and repetition rate ashigh as GHz.

High harmonic generation can be achieved using carbon nano-tubessaturable absorbers. Passive mode-locked fiber lasers using carbonnano-tubes saturable absorbers (CNT-SA) make an option for high rep ratesources due to their ability to readily generate high harmonics of thefundamental rep rate.

FSO can be used in terrestrial, space and undersea applications.

Conditional path lengths control from splitter to aperture can be animportant parameter. TDM multiplexes can be employed consistent withimplementations of the current subject matter to control the relativetemporal time delay between aperture-to-source paths. Each pulse traincan be controlled using parallel time delay channels. This technique canbe used to control conventional multiple-transmit FSO aperture systemsemploying WDM as well as TDM systems. USPL laser pulse-to-pulse spacingcan be maintained and controlled to precise temporal requirements forboth TDM and WDM systems. The techniques described can be used in TDMand WDM fiber based system. The use of TDM multiplexers as describedherein can be used implement unique encryption means onto thetransmitted optical signal. A complementary TDM multiplexer can beutilized to invert the incoming received signal, and thereby recover theunique signature of the pulse signals. A TDM multiplexer describedherein can be utilized to control WDM pulse character for the purpose ofWDM encryption. A TDM multiplexer can be used in conventional FSOsystems wherein multiple apertures connected to a common source signalare capable of having the temporal delay between pulses controlled tomaintain constant path lengths. A TDM multiplexer can be used for TDMfiber based and FSO based systems. A TDM multiplexer can be an enablingtechnology to control optical pulse train relationship for USPL sources.A TDM multiplexer can be used as an atmospheric link characterizationutility across an optical link through measurement of neural correctionfactor to get same pulse relational ship.

Any combination of PZ discs can be used in a transmitter and can have aninfinite number of encryption combinations for USPL based systems, bothfiber and FSO based. The timing can run from 10 GigE switches or theequivalent and to build up the USPL to a Terabit/second (or faster) ratewith a Multiplier Photonic chip, and this Terabit/second signal can bemodulated directly from the 10 GigE switch. While operating in a WDMconfiguration, an interface either to a fiber based system or to a FSOnetwork element can be included.

A system can accept an ultrafast optical pulse train and can generate atrain of optical pulses with pulse-width, spectral content, chirpcharacteristics identical to that of the input optical pulse, and with apulse repetition rate being an integral multiple of that of the inputpulse. This can be accomplished by tapping a fraction of the input pulsepower in a 2×2 optical coupler with an actively controllable opticalcoupling coefficient, re-circulating this tapped pulse over one roundtrip in an optical delay line provided with optical amplification,optical isolation, optical delay (path length) control, optical phaseand amplitude modulation, and compensation of temporal and spectralevolution experienced by the optical pulse in the optical delay line forthe purpose of minimizing temporal pulse width at the output of thedevice, and recombining this power with the 2×2 optical coupler.

Passive or active optical delay control can be used, as can optical gainutilizing rare-earth-doped optical fiber and/or rare-earth-dopedintegrated optical device and/or electrically- or optically-pumpedsemiconductor optical amplification. Dispersion compensation can beprovided using fiber-Bragg gratings and/or volume Bragg gratings.Wavelength division multiplexing data modulation of the pulse traversingthe delay line can be sued as can pulse code data modulation of thepulse traversing the delay line.

The tailoring of conventional USPL sources through synthesis of USPLsquare wave pulses can be accomplished utilizing micro-lithographicamplitude and phase mask technologies, for FSO applications. The abilityto adjust pulse widths using technology and similar approaches tocontrol and actively control pulse with this technology can improvepropagation efficiency through FSO transmission links, thereby improvingsystem availability and received optical power levels.

Active programmable pulse shapers can be used to actively control USPLpulse-width can include matching real-time atmospheric conditions tomaximize propagation through changing environments. One or more of thefollowing techniques can be used in FSO applications to adapt theoptical temporal spectrum using techniques: Fourier Transform Pulseshaping, Liquid Crystal Modular (LCM) Arrays, Liquid Crystal on Silicon(LCOS) Technology, Programmable Pulse Shaping using Acousto-opticmodulators (AOM), Acousto-optic Programmable Dispersive Filter (AOPDF),and Polarization Pulse Shaping.

FIG. 32 shows a process flow chart 3200 illustrating features of amethod, one or more of which can appear in implementations of thecurrent subject matter. At 3202, a beam of light pulses each having aduration of approximately 1 nanosecond or shorter is generated. At 3204,a modulation signal is applied to the beam to generate a modulatedoptical signal. The modulation signal carrying data for transmission toa remote receiving apparatus. The modulated optical signal is receivedat an optical transceiver within an optical communication platform at3206, and at 3210 the modulated optical signal is transmitted using theoptical transceiver for receipt by the second optical communicationapparatus

FIG. 33 shows another process flow chart 3300 illustrating features of amethod, one or more of which can appear in implementations of thecurrent subject matter. At 3302, a beam of light pulses each having aduration of approximately 1 nanosecond or shorter is generated, forexample using a USPL source. The beam of light pulses is transmitted at3304 toward a target atmospheric region via an optical transceiver. At3306, optical information received at the optical transceiver as aresult of optical backscattering of the beam of light pulses from one ormore objects in the target atmospheric region is analyzed.

FIG. 34 shows another process flow chart 3400 illustrating features of amethod, one or more of which can appear in implementations of thecurrent subject matter. At 3402, first and second beams comprising lightpulses are generated, for example by a USPL source. At 3404, a firstmodulation signal is applied to the first beam to generate a firstmodulated optical signal and a second modulation signal is applied tothe second beam to generate a second modulated optical signal. A firstpolarization state of the first modulated optical signal is adjusted at3406. Optionally, a second polarization states of the second modulatedoptical signal can also be adjusted. At 3410, the first modulatedoptical signal having the adjusted first polarization state ismultiplexed with the second modulated signal. At 3412, the multiplexedfirst modulated optical signal having the adjusted first polarizationstate with the second modulated signal is transmitted by an opticaltransceiver for receipt by a second optical communication apparatus.

FIGS. 35A and 35B show exemplary nodes that can be used for transmittingand/or receiving information. Transmit node 3510 and receiving node 3530may be communications platforms as described above, including withreference to FIGS. 1-9 . Additionally, while transmit node 3510 is shownwith components for generating and transmitting a data-bearing opticalsignal, and while receiving node 3530 is shown with components forreceiving and extracting data from an optical signal, these componentsmay be combined in a single node configured to both transmit and receiveoptical signals. In some embodiments, for example, a telescope 3522 mayact as both an aperture for transmitting and receiving optical signals.

FIG. 35A shows an exemplary transmit node 3510. In some embodiments,transmit node 3510 may include a source 3512. In some embodiments, thesource 3512 may be an USPL source, superluminescent diode, or othersource. In other embodiments, the source 3512 may be a continuous wavesource. Preferably, the source 3512 may be configured to generate a beamof light pulses, in which each pulse has a coherence length of less than400 microns. The coherence length of the source is determined as:

${L = {C\frac{\lambda^{2}}{\Delta\lambda}}},$where C is a shaping constant equal to ½, λ is the central wavelength ofthe pulse, and Δλ is the full width at half maximum (FWHM) spectralwidth of the pulse. In some embodiments, the coherence length may beless than 1 mm, less than 600 microns, less than 400 microns, less than200 microns, less than 100 microns, less than 50 microns, or less than 1micron. In embodiments where a continuous wave source is used, thesevalues may refer to the coherence length of the continuous wave beam,rather than that of the pulses.

In some embodiments, the source 3512 may have a central wavelength inthe infrared range. For example, the central wavelength of the source3512 may be between 1400 nm and 1700 nm. In some embodiments, the source3512 may be configured to output pulses at a repetition rate of at least50 MHz, 100 MHz, 200 MHz, 500 MHz, 800 MHz, 1 GHz, 1.25 GHz, 1.5 GHz, 2GHz, 5 GHz, or 10 GHz. The source 3512 may include (internally orexternally) a pulse multiplier, as generally described above, includingwith reference to FIGS. 15 and 18-20 . In some embodiments, the pulsewidth may be less than 10 ns, less than 1 ns, less than 500 ps, lessthan 300 ps, less than 100 ps, less than 50 ps, less than 10 ps, lessthan 1 ps, less than 700 fs, less than 500 fs, less than 300 fs, lessthan 200 fs, or less than 100 fs.

Transmit node 3510 may optionally include a splitter 3514. Splitter 3514may be configured to split pulses from source 3512 into a plurality ofseparated pulses having different wavelength bands. For example, a pulsehaving an original spectral width of 1500-1600 nm could be split intotwenty-five pulses, each having a respective spectral width of 4 nm from1500 nm to 1600 nm (e.g., 1500-1504 nm, 1504-1508 nm, 1508-1512 nm, andso on). Splitter 3514 may use any known beam-splitting mechanism. Eachof the plurality of separated pulses may have coherence lengths of lessthan 1 mm, less than 600 microns, less than 400 microns, less than 200microns, less than 100 microns, less than 50 microns, or less than 1micron.

Transmit node 3510 may include one or more modulators 3516. In someembodiments, each of the modulators 3516 may be a Mach-Zehnder Modulator(MZM). The modulators 3516 may receive a data signal indicating data tobe transmitted in an optical beam, and based on that data signal, mayencode the data into the pulses of the beam using on-off keying or othermodulation techniques. In some embodiments, the modulators 3516 mayallow pulses to pass to indicate a ‘1’ and may block or reduce theamplitude of a pulse to indicate a ‘0’ in a bit stream. In embodimentswhere the beam is split, each of a plurality of separated pulses may bedirected to a respective modulator 3516 of a plurality of modulators. Inother embodiments, each of the plurality of separated pulses may bemodulated by a single modulator 3516. For example, the separated pulsesmay be delayed and staggered in time relative to one another, and themodulator 3516 may encode data into each pulse at a higher repetitionrate than the pulse-generating repetition rate of the source. In a casewhere the source 3512 generates pulses at a rate of at least 1 GHz, forexample, the splitter may split each pulse into twenty-five or moreseparated pulses, which can be modulated by one or more modulators 3516to encode data at a rate of at least 25 Gbps. In some embodiments, thesource may generate pulses at a rate of at least 1 GHz, and the splittermay split each pulse into at least ten, at least twenty, at thirty, atleast forty, or at least fifty separated pulses, to produce data ratesof at least 10 Gbps, at least 20 Gbps, at least 30 Gpbs, at least 40Gpbs, or at least 50 Gbps. In some embodiments, the FWHM bandwidth ofthe source may be at least 100 nm, at least 150 nm, or at least 200 nm,which may allow pulses to be split into more separated pulses withoutreducing the coherence length of those pulses below the values describedbelow with respect to FIGS. 40 and 41 .

After being modulated, the pulses (optionally, the separated pulses inthe case where a splitter is used) may be passed to an optionalthresholding filter 3518. In some embodiments, the thresholding filtermay be a saturable absorber (or a different nonlinear device) thatattenuates weak pulses and transmits strong pulses. The thresholdingfilter 3518 may be configured to eliminate or substantially diminishpulses below a defined threshold, while allowing pulses above thatthreshold to pass. In some embodiments, modulator 3516 may significantlydiminish pulses where a “0” is intended to be transmitted, but it may beimperfect and some amount of optical energy may pass through, which,when amplified by amplifier 3520, could produce signals strong enough togenerate bit errors. By using a thresholding filter 3518, pulses thatare intended to be eliminated may be more fully eliminated, therebyimproving the system's data transmission accuracy.

The modulated pulses may be passed to an amplifier 3520, which mayincrease the magnitude of the pulses for transmission by telescope 3522(which may be, for example, an aperture and/or lens). In cases where asplitter is used, the separated pulses may be recombined using arecombiner (not shown) before or after being passed to amplifier 3520.

FIG. 35B shows an exemplary embodiment of a receiving node 3530, whichmay be configured to receive and extract data from an optical beamtransmitted by, e.g., a transmit node 3510. Receiving node 3530 mayinclude an aperture 3532, an optional splitter 3534, and one or morephotoreceivers 3536, which may have specific characteristics in relationto the source, as described in detail below. The photoreceivers 3536 mayinclude a photodiode and processing circuitry. In some embodiments, thephotoreceivers 3536 may be, for example, an avalanche photodiode. Insome embodiments, the processing circuitry of a photoreceiver maydetermine whether received light in a detection window exceeds adetection threshold and output bit data (e.g., a ‘0’ or ‘1’) for thatwindow based on the result of that determination. Receiving node 3530may be an optical communications platform as described above. In someembodiments, the components of transmit node 3510 and receiving node3530 may be included in a single transceiver node.

Aperture 3532 may be configured to receive an optical signal, such as anoptical beam transmitted by a transmit node 3510 as described in FIG.35A. In some embodiments, the light received at aperture 3532 may passthrough a filter that screens wavelengths of light that are not near thecenter wavelength of the source. For example, the source in the transmitnode may have a center wavelength between 1500 nm and 1700 nm, and thefilter at the receiving node 3530 may block or reduce light outside ofthe source band. For example, the filter may reduce a magnitude of lightbelow 1500 nm. Optionally, the filter may additionally block longerwavelengths of light, or the threshold may be set at lower wavelengths,such as at 1480 nm or 1460 nm. Optionally, receive node 3530 may includea splitter 3534, which may split pulses in a received beam into aplurality of separated pulses of different wavelength bands. In a casewhere the pulses are split and separately modulated at the transmit node3510, the pulses may be split into the same wavelength bands by thesplitter 3534 in the receive node. The pulses (combined pulses orseparated pulses, in the case where a splitter is used) may then beprocessed by one or more photoreceivers 3536. In embodiments where apulse is split into a plurality of separated pulses, each pulse may bedirected to a respective photoreceiver, which may be configured todetermine whether an “on” or “off” signal was transmitted in a givendetection window. In some embodiments, encoding modalities other thanon-off keying may be used, such as frequency modulation. Additionaldetail regarding photoreceivers 3536 is provided below with respect toFIG. 41 .

FIG. 36 shows an exemplary arrangement in which data is transmitted froma first communications network 3542 to a second communications network3544 over an optical communication distance D using a transmit node 3510and a receiving node 3530, such as those described above with respect toFIGS. 35A-35B. Data may be received from optical communications network3542 encoded into an optical beam and transmitted across opticalcommunications distance D using transmit node 3510. Receiving node 3530may receive the optical beam, extract the transmitted data, and pass thedata to communications network 3544. In some embodiments, data fromcommunications 3544 may also be transmitted from node 3530 back to node3510, which may pass that data to communications network 3542 to enabletwo-way communication. In some embodiments, optical communicationdistance may be at least 0.5 miles, at least 1 mile, at least 2 miles,at least 3 miles, at least 5 miles, at least 7 miles, at least 10 miles,or at least 20 miles.

FIG. 37 shows an exemplary beam traveling over an optical communicationdistance D, such as 1 mile, through a perfectly uniform refractive indexmedium. Even in a medium of perfectly constant index of refraction, thebeam will spread naturally due to diffraction, however the beam remainsthe same shape and simply expands by an amount that is proportional tothe propagation distance, and there are no beam scintillation effects ina uniform index of refraction medium.

FIG. 38 provides a diagrammatic representation of photons in a beamtraveling through a variably refractive medium. The atmosphere hasfluctuations in temperature, density, pressure, humidity, aerosols,wind, convection, and other parameters, which causes a refractive indexof the atmosphere to vary. As an optical beam travels through theatmosphere or other variably refractive medium such as water, photonswithin the beam may be refracted slightly differently than otherphotons. As shown in FIG. 38 , different ray paths within the beam maybe refracted differently due to variations in the refractive index inthe variably refractive medium. As a result, in a system such as thatshown in FIG. 35 where a free space optical beam is transmitted over asufficiently large optical communication distance D and received at areceiving node, different photons within a single pulse may take pathsof different lengths to reach the receiving node and may arrive atdifferent times. These differences in path length, and the time requiredfor a photon to travel these distances, can produce coherentinterference and diminish signal quality in a free space opticalcommunications system if the time delays are less than the coherencelength of the source. Solutions for this problem are described herein,including with reference to FIGS. 40 and 41 and as applied within asystem such as those shown in FIGS. 35A, 35B, and 36 .

In addition to variance in path length, photons in a pulse may travel atvariable speeds to due to variations in atmospheric conditions,including humidity, temperature, and density. Because different photonsin a pulse travel though slightly different atmospheric conditions, thephotons may travel at different speeds and arrive at different times.Additionally, different wavelengths of light within a pulse may travelat different speeds, which can further broaden a pulse as it travelsthrough a variably refractive medium.

FIG. 39 shows a diagrammatic representation of a pulse as launched by atransmitter and as received by a photoreceiver. As shown in FIG. 39 ,the pulse may have a 90 femtosecond pulse width when it is transmittedby a transmit node. The pulse may then travel over an opticaltransmission distance where it may be received by a photoreceptor havinga detection window 4020 of a defined duration, such as 500 picoseconds.When the pulse is received by the photoreceiver, its received pulsewidthmay be broadened by passing through the variably refractive medium, asdescribed above with respect to FIGS. 37-38 . Due to variance in pathlengths traveled by the beams and variance in atmospheric conditionsthrough which the beams travel, different photons may arrive at thedetector at different times according to a distribution curve, which mayhave a temporal duration that is longer than the pulse duration atlaunch. The amount of broadening can vary depending on the length of theoptical communication distance and atmospheric conditions, includinghumidity, temperature, density, and the presence of aerosols such asfog. This broadening can be the order of picoseconds or more in someconditions.

The pulse may have a temporal distribution curve as shown. While anormal temporal distribution curve is shown, other pulse shapes arepossible. By making the width of the curve 4010 longer (e.g., 3× longer)than the coherence length of pulses that are launched, coherent beaminterference and coherent beam scintillation may be reduced.

FIG. 40 shows an exemplary temporal distribution curve of ashort-duration (e.g., approximately 100 femtosecond) pulse 4010 thattraveled a substantial distance (e.g., one mile) through a variablyrefractive medium and been temporally broadened. The pulse, as itarrives at the photoreceiver, may have a FWHM duration 4030 and acoherence time 4040, which may be equal to a coherence length of thepulse divided by the speed of light through the variably refractivemedium. In some embodiments, the FWHM duration 4030 may be greater thanthe coherence time 4040 of the pulse. Preferably, the FWHM duration 4030may be at least 2×, at least 3×, at least 4×, at least 5×, at least 6×,at least 8×, at least 10×, or at least 12× the coherence time 4040 ofthe pulse. By ensuring that the FWHM duration 4030 of the pulse asreceived at the photoreceiver is relatively large as compared to thecoherence time 4040 of the pulse 4010, interference between thedifferent ray paths of the pulse as they arrive at the photoreceivers atdifferent times may be reduced, and a signal with reduced noise andhigher quality may arrive at the photoreceiver.

The photoreceiver may have a detection window 4020 of a specifiedduration. A shorter detection window generally allows higher datathroughput. For example, in a system that uses on-off keying for datamodulation, a photoreceiver having a detection window of 1 nanosecondcan extract up to 1 Gbps while a photoreceiver having a detection windowof 100 picoseconds can extract up to 10 Gbps. The photoreceiver may haverepeating detection windows of less than 100 ns, less than 10 ns, lessthan 1 ns, less than 100 ps, or less than 10 ps.

Pulse length and temporal broadening can, however, cause photons from apulse intended to be received in one detection window to fall into anadjacent detection window. In the case where the adjacent detectionwindow should not receive transmitted photons (e.g., because a ‘0’ istransmitted in that bit position), this phenomenon can produce biterrors. Accordingly, to maximize data transmission accuracy, it isimportant that the FWHM duration 4030 of the pulse as received at thephotoreceiver be greater (and preferably at least three times as large)than the coherence length 4040 of the pulse, while at the same time, theFWHM duration 4030 of the pulse as received at the photoreceiver shouldalso be substantially less than the detection window 4020 of thephotoreceiver.

For example, the detection window 4020 may be at least 2×, at least 5×,at least 6×, at least 7×, at least 8×, at least 10×, or at least 20× aslarge as the FWHM duration 4030 of the pulse as received at thephotoreceiver. Preferably, at least 95%, at least 99%, or at least99.99% of the photons in a pulse that arrive at the photoreceiver mayarrive at a respective arrival time that is spaced from a center 4040 ofthe temporal distribution curve of the pulse by a respective timedifference that is less than half of the detection window duration ofthe photoreceiver. Note that although the center 4040 of the temporaldistribution curve of the pulse is shown at the center of the detectionwindow 4020, this need not be the case, and pulses may arrive earlier orlater than the midpoint of a detection window. It may be preferable thatthe center 4040 of the temporal distribution curve be at or near thecenter of the detection window 4020 to reduce the potential for photonsin a pulse to spill over into an adjacent detection window. In someembodiments, the center 4040 of the temporal distribution curve may beless than 100 picoseconds, 50 picoseconds, 20 picoseconds, 10picoseconds, 5 picoseconds, 1 picosecond, 800 femtoseconds, or 500femtoseconds from the center of the detection window 4020.

By specifying relationships between the coherence time 4040 of thepulse, the FWHM duration 4030 of the pulse as it arrives at thephotoreceiver, and the detection window 4020 of the photoreceiver in themanner described herein, data transmission accuracy and effectivetransmission range can be greatly improved (see below discussion withrespect to FIG. 42 for test results). The FWHM duration 4030 of thepulse as it arrives at the photoreceiver may vary depending on the pulselength as transmitted from the source, the medium through which thepulse travels (e.g., atmospheric pressure, temperature, sunlightintensity, aerosols), and the distance over which the pulse travels toreach the photoreceiver. Accordingly, the coherence time 4040 of thepulse may need to be decreased and/or the detection window 4020 of oneor more photoreceivers may need to be increased depending on conditionsfor the optical communication system. Decreasing coherence time 4040 andincreasing detection window 4020 may thus improve data transmissionquality while negatively impacting data throughput. In some embodiments,the system may be configured to determine a data transmission quality ofthe system (e.g., a bit error rate or a measurement of signal valuesabove or below a detection threshold), and in response to the determineddata transmission quality, modify either or both of the coherence time4040 of the pulse or the detection window duration 4020 of thephotoreceiver.

Similarly, when using a source that can continuously emit light, such asa continuous wave source or a superluminescent diode, the emitted lightcan be gated into pulses (or otherwise converted into pulses using datamodulation or other known techniques) that occupy only a relativelysmall fraction of the duration of the detection window, and those pulsesmay be timed to arrive at or near the centers of the detection windowsof the photoreceiver. Gating and timing the pulses in this manner canreduce the risk that photons in an “on” window (where light is intendedto be transmitted) may spill over into an “off” window (where light isnot intended to be transmitted) and produce bit errors. The pulsedurations and positions relative to the detection windows describedabove may thus also apply to pulses generated using sources that cancontinuously emit light. In such cases, although the sources cancontinuously emit light, the effective output may be “off” for amajority of the time even during “on” transmission windows where lightis intended to be transmitted, so that sufficient space may be leftbetween the center of the pulse and the ends of the detection window toavoid spillover. For example, during an “on” bit window where light isintended to be transmitted, the effective output from the continuousemission source may be “on” less than 50%, less than 30%, less than 20%,or less than 10% of the respective transmission bit window.

FIG. 41 shows a diagrammatic representation of light pulses arriving indetection windows 4020 a, 4020 b, 4020 c of a photoreceiver. The lightpulses may be of any shape and generally may be broadened to some extentby traveling over an optical communication distance through a variablyrefractive medium. In a first detection window 4020 a, a light pulse mayarrive at or near the center of the window and may cause the totalreceived light in that window to exceed a detection threshold Vth, whichmay be processed by circuitry of the photoreceiver to indicate that apulse was received in that window. In some embodiments, this may causethe photoreceiver to output a ‘1’ for this detection window. At the endof detection window 4020 a and before detection window 4020 b, thephotoreceiver circuit may be reset and return to zero. In detectionwindow 4020 b, no pulse is transmitted (e.g., because a ‘0’ is intendedto be transmitted and a modulator at the transmit node blocked thepulse), and the total light received in window 4020 b may be below thedetection threshold Vth. This may cause the photoreceiver to output a‘0’ for this detection window. The photoreceiver circuit may again bereset and return to zero, and the cycle may repeat with a third window4020 c, and so on.

The detection threshold Vth may be configured so that it is sufficientlyhigh that environmental light will not trigger a false positive butsufficiently low that true pulses will reliably exceed the detectionthreshold Vth. It is important that pulses sufficiently exceed a noisefloor so that there is sufficient signal difference between “on” and“off” bit windows so that the detection threshold Vth may be both highenough to ignore environmental noise but low enough to capture everytransmitted pulse. This is particularly challenging over longerdistances (e.g., a mile or more) and in suboptimal environmentalconditions (e.g., partly sunny, significant aerosols). The relationshipsbetween pulse length at the photoreceiver, coherence time, and detectionwindow described herein with respect to FIGS. 39-41 greatly improvesignal quality transmission and allow effective detection thresholds Vtheven for free space optical systems transmitting data over opticaltransmission distances in excess of 1 mile, 2 miles, 3 miles, 5 miles,or 7 miles.

In a case with a beam splitter and multiple photoreceivers, each of themultiple photoreceivers may generate a bit stream based on the separatedpulses that are directed to that photoreceiver, and the bit streams fromthe respective photoreceivers may be interleaved to produce a combinedbit stream having a higher data rate. The combined bit stream may beoutputted to a communication network as described above, including withrespect to FIG. 36 .

FIG. 42 shows an example of test data received over a one-mile opticalcommunication distance. The test data compares optical signals generatedusing a transmit node as described above with respect to FIG. 35Aagainst optical signals generated using a continuous wave source havingthe same average power as the USPL source. Specifically, to generate thedata shown in the top row of the chart shown in FIG. 42 , a USPL sourceincorporated in a transmit node as described above with respect to FIG.35A was used to transmit data over an optical communication distance ofone mile. The received signal was directed at a piece of white paper,and an infrared camera was placed behind the paper to record the lightthat passed through the paper. To generate the data shown in the bottomrow of the chart shown in FIG. 42 , the same experimental setup was usedwith a continuous wave source having the same average power and sameoptical communication distance as the USPL source. The light from boththe USPL source and the CW source was directed at the same sheet ofwhite paper, and the two signal spots were captured in the same frameusing the infrared camera. The spot sizes were approximately 12 inchesin diameter. Background environmental light was subtracted from eachpixel, and a each pixel was subjected to a thresholding logic such thatpixels in which the received optical signal was above the threshold wereset to “white” and pixels in which the received optical signal was belowthe threshold were set to “black.” The four images shown for each sourcewere taken from the same frames in the video feed, and those frames wereequally spaced at intervals of 10 seconds. Frame A shows the receivedsignals from the USPL and CW sources at 10 seconds, Frame B shows theshows the received signals from the USPL and CW sources at 20 seconds,Frame C shows the shows the received signals from the USPL and CWsources at 30 seconds, and Frame D shows the shows the received signalsfrom the USPL and CW sources at 40 seconds.

This data shows that the transmit node as described herein producesultrashort pulses that are substantially more clustered and, within thedetection field, much more reliably exceed the detection threshold. Asapplied to a communication system using a photoreceiver having thecharacteristics described above, including with reference to FIGS. 35Bto 41 , this produces vastly improved data transmission accuracy.Applicant's testing of systems in accordance with this description hasdemonstrated free space optical communication distances in excess of 1mile, 2 miles, 3 miles, 5 miles, and up to as much as 7.4 miles withzero bit error rate as measured over time intervals of at least 10seconds, at least 30 seconds, at least 60 seconds, at least 10 minutes,at least 30 minutes, and at least 1 hour. In some embodiments, systemsdescribed herein may transmit data over an optical communicationdistance of at least one mile and have a measured bit error rate of lessthan one in one million, less than one in one billion, less than one inone trillion, or less than one in one quadrillion over a measurementperiod of at least sixty seconds. To Applicant's knowledge, no otherfree space optical system has achieved similarly low over opticalcommunication distances of even one half of one mile.

Thus, the systems described herein allow for substantially improved datatransmission accuracy, communication link distance, and they also allowfree space optical communication to be used in inclement environmentalconditions (e.g., rain, fog, atmospheric scintillation) that, in priorsystems, rendered free space optical communication ineffective. In someembodiments, the improved data transmission quality and range may alsoallow for free space optical communication to be applied to systems thatwould have previously been impossible to use effectively. For example, atransmit node and/or receiving node in accordance with the presentdisclosure may be provided in an Earth-orbiting satellite to provide forground-to-space and/or space-to-ground free space optical communication.Due to the amount of atmosphere that a beam must travel between Earth'sground level and space, effective optical data transmission has not beendemonstrated using technologies prior to the present disclosure, but thetechnology described herein can achieve effective optical communicationover this distance.

FIG. 43 shows an exemplary ranging node 4400 that can be used to detectobjects or surfaces and determine positions of those objects relative tothe node. The ranging node 4400 may generally include the components ofthe transmit and receiving nodes 3510, 3530 described above with respectto FIGS. 35A and 35B. For example, the ranging node 4400 may include asource 3512, a splitter, one or more modulators, an amplifier, and atelescope. These elements may collectively be configured to emit opticalpulses that travel through a variably refractive medium toward a surfaceS. In the case of a laser ranging node, data modulation is optional butmay be included to encode information relating to the pulses, nodes, orother information. Photons from the optical pulses may be reflected bysurface S and return to the node 4400. The total travel distance of theoptical pulses from transmission by the ranging node to receipt of thereflected pulse may be twice the distance of the node to the surface S.Upon return to the node, the pulses may be received by an aperture 3532,optionally split by a splitter 3534, and analyzed using one or morephotoreceivers 3536. Each of these components may have the sameproperties and parameters as the corresponding components describedabove with respect to FIGS. 35A to 41 . Ranging node 4400 mayadditionally include a time-of-flight (TOF) circuit 4410, which may beconfigured to determine the time of flight of a pulse to reach surface Sand return to node 4400, and thereby determine a distance of thatsurface S from the ranging node 4400.

One or more aspects or features of the subject matter described hereincan be realized in digital electronic circuitry, integrated circuitry,specially designed application specific integrated circuits (ASICs),field programmable gate arrays (FPGAs) computer hardware, firmware,software, and/or combinations thereof. These various aspects or featurescan include implementation in one or more computer programs that areexecutable and/or interpretable on a programmable system including atleast one programmable processor, which can be special or generalpurpose, coupled to receive data and instructions from, and to transmitdata and instructions to, a storage system, at least one input device,and at least one output device.

These computer programs, which can also be referred to as programs,software, software applications, applications, components, or code,include machine instructions for a programmable processor, and can beimplemented in a high-level procedural and/or object-orientedprogramming language, and/or in assembly/machine language. As usedherein, the term “machine-readable medium” refers to any computerprogram product, apparatus and/or device, such as for example magneticdiscs, optical disks, memory, and Programmable Logic Devices (PLDs),used to provide machine instructions and/or data to a programmableprocessor, including a machine-readable medium that receives machineinstructions as a machine-readable signal. The term “machine-readablesignal” refers to any signal used to provide machine instructions and/ordata to a programmable processor. The machine-readable medium can storesuch machine instructions non-transitorily, such as for example as woulda non-transient solid-state memory or a magnetic hard drive or anyequivalent storage medium. The machine-readable medium can alternativelyor additionally store such machine instructions in a transient manner,such as for example as would a processor cache or other random accessmemory associated with one or more physical processor cores.

To provide for interaction with a user, one or more aspects or featuresof the subject matter described herein can be implemented on a computerhaving a display device, such as for example a cathode ray tube (CRT) ora liquid crystal display (LCD) or a light emitting diode (LED) monitorfor displaying information to the user and a keyboard and a pointingdevice, such as for example a mouse or a trackball, by which the usermay provide input to the computer. Other kinds of devices can be used toprovide for interaction with a user as well. For example, feedbackprovided to the user can be any form of sensory feedback, such as forexample visual feedback, auditory feedback, or tactile feedback; andinput from the user may be received in any form, including, but notlimited to, acoustic, speech, or tactile input. Other possible inputdevices include, but are not limited to, touch screens or othertouch-sensitive devices such as single or multi-point resistive orcapacitive trackpads, voice recognition hardware and software, opticalscanners, optical pointers, digital image capture devices and associatedinterpretation software, and the like. A computer remote from ananalyzer can be linked to the analyzer over a wired or wireless networkto enable data exchange between the analyzer and the remote computer(e.g. receiving data at the remote computer from the analyzer andtransmitting information such as calibration data, operating parameters,software upgrades or updates, and the like) as well as remote control,diagnostics, etc. of the analyzer.

While the subject matter of this disclosure has been described and shownin considerable detail with reference to certain illustrativeembodiments, including various combinations and sub-combinations offeatures, those skilled in the art will readily appreciate otherembodiments and variations and modifications thereof as encompassedwithin the scope of the present disclosure. Moreover, the descriptionsof such embodiments, combinations, and sub-combinations are not intendedto convey that the claimed subject matter requires features orcombinations of features other than those expressly recited in theclaims. Accordingly, the scope of this disclosure is intended to includeall modifications and variations encompassed within the spirit and scopeof the following appended claims.

The invention claimed is:
 1. An optical communication system foroptically transmitting data through a variably refractive medium, thesystem comprising: an optical source configured to generate a beamcomprising a series of light pulses each having a duration of less than100 picoseconds; a modulator configured to modulate the series of lightpulses in response to a data transmission signal, thereby encodingtransmission data into the series of light pulses; a photoreceiver, thephotoreceiver having: a detection window duration of less than 1nanosecond; and a detection threshold, wherein the photoreceiver isconfigured to indicate whether a received optical energy during a givendetection window is greater than the detection threshold; wherein: theseries of light pulses comprises a first light pulse having a coherencelength of less than 400 microns; when the first pulse travels throughthe variably refractive medium, photons in the first pulse are refractedto travel along different ray paths having different lengths to thephotoreceiver; the photons of the first pulse arrive at thephotoreceiver according to a temporal distribution curve that depends,at least in part, on the duration of the first pulse and the lengths ofthe different ray paths taken by the photons in the first pulse to thephotoreceiver; a full width at half maximum (FWHM) value of the temporaldistribution curve is at least three times as large as a coherence timevalue equal to the coherence length of the first pulse divided by thespeed of light through the variably refractive medium; and the detectionwindow of the photoreceiver is at least six times as large as the FWHMvalue of the temporal distribution curve.
 2. The optical communicationsystem of claim 1, wherein the optical source and the photoreceiver arespaced by a free space optical communication distance of at least onemile, and the optical communication system has a measured bit error rateof less than one in one billion over the free space opticalcommunication distance of at least one mile for a measurement period ofat least sixty seconds.
 3. The optical communication system of claim 1,wherein the FWHM value of the temporal distribution curve is at leastsix times as large as the coherence time value equal to the coherencelength of the first pulse divided by the speed of light through thevariably refractive medium.
 4. The optical communication system of claim1, wherein the FWHM value of the temporal distribution curve is at leastten times as large as the coherence time value equal to the coherencelength of the first pulse divided by the speed of light through thevariably refractive medium.
 5. The optical communication system of claim1, wherein at least 95% of the photons of the first pulse that arrive atthe photoreceiver arrive at a respective arrival time that is spacedfrom the center of the temporal distribution curve by a respective timedifference that is less than half of the detection window duration ofthe photoreceiver.
 6. The optical communication system of claim 1,wherein the optical source is located on a ground station and thephotoreceiver is disposed on an earth-orbiting satellite, and theoptical communication system has a measured bit error rate of less thanone in one billion over a free space optical communication distancebetween the ground station and the earth-orbiting satellite for ameasurement period of at least sixty seconds.
 7. The opticalcommunication system of claim 1, wherein the series of light pulsesgenerated by the optical source has a center wavelength between 1500 nmand 1700 nm, and the photoreceiver is disposed on a detection node thatincludes a filter configured to reduce an amount of light having awavelength below 1500 nm that reaches the photoreceiver.
 8. The opticalcommunication system of claim 1, wherein: the optical source is disposedin a transmit node; the transmit node comprises a beam splitterconfigured to split a combined pulse generated by the optical sourceinto a plurality of separated pulses having different wavelength bands,the first pulse being among the plurality of separated pulses; thetransmit node is configured to separately modulate each of the pluralityof separated pulses in response to the data transmission signal, therebyencoding the transmission data into the plurality of separated pulses;each of the plurality of separated pulses has a respective coherencelength of less than 400 microns; the photoreceiver is disposed in areceiving node; the receiving node comprises a beam splitter configuredto direct the plurality of separated pulses to a respectivephotoreceiver of a plurality of photoreceivers, the photoreceiver beingamong the plurality of photoreceivers; each of the plurality ofseparated pulses comprises respective ray paths that arrive at therespective photoreceiver of the plurality of photoreceivers according toa respective temporal distribution curve; and each of the plurality ofseparated pulses has a respective FWHM value of its temporaldistribution curve that is at least three times as large as a respectivecoherence time value equal to the respective coherence length of therespective separated pulse divided by the speed of light through thevariably refractive medium.
 9. The optical communication system of claim1, wherein the optical source is an ultrashort pulse laser (USPL) sourceand the duration of the pulses of the series of light pulses is lessthan 500 femtoseconds.
 10. The optical communication system of claim 1,further comprising: an amplifier, the amplifier being configured toamplify a magnitude of the series of light pulses; and a thresholdingfilter, the thresholding filter being configured to receive the seriesof light pulses after the transmission data has been encoded by themodulator and before the series of light pulses reaches the amplifier,wherein the thresholding filter is configured to selectively attenuatepulses having a magnitude less than a threshold of the thresholdingfilter.
 11. A laser ranging system, the system comprising: an opticalsource configured to generate a beam comprising a series of light pulseseach having a duration of less than 100 picoseconds; a photoreceiver,the photoreceiver having: a detection window duration of less than 1nanosecond; and a detection threshold, wherein the photoreceiver isconfigured to indicate whether a received optical energy during a givendetection window is greater than the detection threshold; wherein: theseries of light pulses comprises a first light pulse having a coherencelength of less than 400 microns; when the first pulse travels through avariably refractive medium, photons in the first pulse are refracted totravel along different ray paths having different lengths to thephotoreceiver; the photons of the first pulse arrive at thephotoreceiver according to a temporal distribution curve that depends,at least in part, on the duration of the first pulse and the lengths ofthe different ray paths taken by the photons in the first pulse to thephotoreceiver; a full width at half maximum (FWHM) value of the temporaldistribution curve is at least three times as large as a coherence timevalue equal to the coherence length of the first pulse divided by thespeed of light through the variably refractive medium; the detectionwindow of the photoreceiver is at least six times as large as the FWHMvalue of the temporal distribution curve; and the laser ranging systemis configured to transmit the series of light pulses toward a surface,receive at least a portion of the series of light pulses that have beenreflected by the surface, and, based on a time of flight of the receivedportion of the series of light pulses, determine a distance of at leasta portion of the surface from the laser ranging system.
 12. The laserranging system of claim 11, wherein the pulses travel over a totaltravel distance of at least one mile, the total travel distance beingmeasured from the laser ranging system to a surface and then back to thelaser ranging system, and the optical communication system has ameasured bit error rate of less than one in one billion over the freespace optical communication distance of at least one mile for ameasurement period of at least sixty seconds.
 13. The laser rangingsystem of claim 11, wherein the FWHM value of the temporal distributioncurve is at least six times as large as the coherence time value equalto the coherence length of the first pulse divided by the speed of lightthrough the variably refractive medium.
 14. The laser ranging system ofclaim 11, wherein the FWHM value of the temporal distribution curve isat least ten times as large as the coherence time value equal to thecoherence length of the first pulse divided by the speed of lightthrough the variably refractive medium.
 15. The laser ranging system ofclaim 11, wherein at least 95% of the photons of the first pulse thatarrive at the photoreceiver arrive at a respective arrival time that isspaced from the center of the temporal distribution curve by arespective time difference that is less than half of the detectionwindow duration of the photoreceiver.
 16. The laser ranging system ofclaim 11, wherein the optical source and the photoreceiver are locatedon a ground station and the surface is disposed on an earth-orbitingsatellite.
 17. The laser ranging system of claim 11, wherein the seriesof light pulses generated by the optical source has a center wavelengthbetween 1500 nm and 1700 nm, and the photoreceiver is disposed behind afilter that is configured to reduce an amount of light having awavelength below 1500 nm that reaches the photoreceiver.
 18. The laserranging system of claim 11, wherein: the laser ranger system comprises afirst beam splitter configured to split a combined pulse generated bythe optical source into a plurality of separated pulses having differentwavelength bands, the first pulse being among the plurality of separatedpulses; the laser ranging system is configured to separately modulateeach of the plurality of separated pulses in response to a datatransmission signal, thereby encoding transmission data into theplurality of separated pulses; each of the plurality of separated pulseshas a respective coherence length of less than 400 microns; the laserranging system comprises a second beam splitter configured to direct theplurality of separated pulses to a respective photoreceiver of aplurality of photoreceivers, the photoreceiver being among the pluralityof photoreceivers; each of the plurality of separated pulses comprisesrespective ray paths that arrive at the respective photoreceiver of theplurality of photoreceivers according to a respective temporaldistribution curve; and each of the plurality of separated pulses has arespective FWHM value of its temporal distribution curve that is atleast three times as large as a respective coherence time value equal tothe respective coherence length of the respective separated pulsedivided by the speed of light through the variably refractive medium.19. The laser ranging system of claim 11, wherein the optical source isan ultrashort pulse laser (USPL) source and the duration of the pulsesof the series of light pulses is less than 500 femtoseconds.